Add lint to remind you not to leave an focused/exclusive test in

Relevant rule: https://github.com/jest-community/eslint-plugin-jest/blob/ad04fcc97b5dcc676c5bd2152f84608c08e0c50a/docs/rules/no-focused-tests.md

Spawning from #2856 (comment)

.eslintrc.js

@@ -6,6 +6,7 @@ module.exports = {
     extends: [
         "plugin:matrix-org/babel",
         "plugin:import/typescript",
+        "plugin:jest/recommended",
     ],
     env: {
         browser: true,

package.json

@@ -100,6 +100,7 @@
     "eslint-config-google": "^0.14.0",
     "eslint-import-resolver-typescript": "^3.5.1",
     "eslint-plugin-import": "^2.26.0",
+    "eslint-plugin-jest": "^27.1.4",
     "eslint-plugin-matrix-org": "^0.7.0",
     "eslint-plugin-unicorn": "^44.0.2",
     "exorcist": "^2.0.0",

yarn.lock

@@ -1853,6 +1853,14 @@
     "@typescript-eslint/types" "5.42.0"
     "@typescript-eslint/visitor-keys" "5.42.0"
 
+"@typescript-eslint/scope-manager@5.42.1":
+  version "5.42.1"
+  resolved "https://registry.yarnpkg.com/@typescript-eslint/scope-manager/-/scope-manager-5.42.1.tgz#05e5e1351485637d466464237e5259b49f609b18"
+  integrity sha512-QAZY/CBP1Emx4rzxurgqj3rUinfsh/6mvuKbLNMfJMMKYLRBfweus8brgXF8f64ABkIZ3zdj2/rYYtF8eiuksQ==
+  dependencies:
+    "@typescript-eslint/types" "5.42.1"
+    "@typescript-eslint/visitor-keys" "5.42.1"
+
 "@typescript-eslint/type-utils@5.42.0":
   version "5.42.0"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/type-utils/-/type-utils-5.42.0.tgz#4206d7192d4fe903ddf99d09b41d4ac31b0b7dca"
@@ -1868,6 +1876,11 @@
   resolved "https://registry.yarnpkg.com/@typescript-eslint/types/-/types-5.42.0.tgz#5aeff9b5eced48f27d5b8139339bf1ef805bad7a"
   integrity sha512-t4lzO9ZOAUcHY6bXQYRuu+3SSYdD9TS8ooApZft4WARt4/f2Cj/YpvbTe8A4GuhT4bNW72goDMOy7SW71mZwGw==
 
+"@typescript-eslint/types@5.42.1":
+  version "5.42.1"
+  resolved "https://registry.yarnpkg.com/@typescript-eslint/types/-/types-5.42.1.tgz#0d4283c30e9b70d2aa2391c36294413de9106df2"
+  integrity sha512-Qrco9dsFF5lhalz+lLFtxs3ui1/YfC6NdXu+RAGBa8uSfn01cjO7ssCsjIsUs484vny9Xm699FSKwpkCcqwWwA==
+
 "@typescript-eslint/typescript-estree@5.42.0":
   version "5.42.0"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/typescript-estree/-/typescript-estree-5.42.0.tgz#2592d24bb5f89bf54a63384ff3494870f95b3fd8"
@@ -1881,6 +1894,19 @@
     semver "^7.3.7"
     tsutils "^3.21.0"
 
+"@typescript-eslint/typescript-estree@5.42.1":
+  version "5.42.1"
+  resolved "https://registry.yarnpkg.com/@typescript-eslint/typescript-estree/-/typescript-estree-5.42.1.tgz#f9a223ecb547a781d37e07a5ac6ba9ff681eaef0"
+  integrity sha512-qElc0bDOuO0B8wDhhW4mYVgi/LZL+igPwXtV87n69/kYC/7NG3MES0jHxJNCr4EP7kY1XVsRy8C/u3DYeTKQmw==
+  dependencies:
+    "@typescript-eslint/types" "5.42.1"
+    "@typescript-eslint/visitor-keys" "5.42.1"
+    debug "^4.3.4"
+    globby "^11.1.0"
+    is-glob "^4.0.3"
+    semver "^7.3.7"
+    tsutils "^3.21.0"
+
 "@typescript-eslint/utils@5.42.0":
   version "5.42.0"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/utils/-/utils-5.42.0.tgz#f06bd43b9a9a06ed8f29600273240e84a53f2f15"
@@ -1895,6 +1921,20 @@
     eslint-utils "^3.0.0"
     semver "^7.3.7"
 
+"@typescript-eslint/utils@^5.10.0":
+  version "5.42.1"
+  resolved "https://registry.yarnpkg.com/@typescript-eslint/utils/-/utils-5.42.1.tgz#2789b1cd990f0c07aaa3e462dbe0f18d736d5071"
+  integrity sha512-Gxvf12xSp3iYZd/fLqiQRD4uKZjDNR01bQ+j8zvhPjpsZ4HmvEFL/tC4amGNyxN9Rq+iqvpHLhlqx6KTxz9ZyQ==
+  dependencies:
+    "@types/json-schema" "^7.0.9"
+    "@types/semver" "^7.3.12"
+    "@typescript-eslint/scope-manager" "5.42.1"
+    "@typescript-eslint/types" "5.42.1"
+    "@typescript-eslint/typescript-estree" "5.42.1"
+    eslint-scope "^5.1.1"
+    eslint-utils "^3.0.0"
+    semver "^7.3.7"
+
 "@typescript-eslint/visitor-keys@5.42.0":
   version "5.42.0"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/visitor-keys/-/visitor-keys-5.42.0.tgz#ee8d62d486f41cfe646632fab790fbf0c1db5bb0"
@@ -1903,6 +1943,14 @@
     "@typescript-eslint/types" "5.42.0"
     eslint-visitor-keys "^3.3.0"
 
+"@typescript-eslint/visitor-keys@5.42.1":
+  version "5.42.1"
+  resolved "https://registry.yarnpkg.com/@typescript-eslint/visitor-keys/-/visitor-keys-5.42.1.tgz#df10839adf6605e1cdb79174cf21e46df9be4872"
+  integrity sha512-LOQtSF4z+hejmpUvitPlc4hA7ERGoj2BVkesOcG91HCn8edLGUXbTrErmutmPbl8Bo9HjAvOO/zBKQHExXNA2A==
+  dependencies:
+    "@typescript-eslint/types" "5.42.1"
+    eslint-visitor-keys "^3.3.0"
+
 "@yarnpkg/lockfile@^1.1.0":
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/@yarnpkg/lockfile/-/lockfile-1.1.0.tgz#e77a97fbd345b76d83245edcd17d393b1b41fb31"
@@ -3391,6 +3439,13 @@ eslint-plugin-import@^2.26.0:
     resolve "^1.22.0"
     tsconfig-paths "^3.14.1"
 
+eslint-plugin-jest@^27.1.4:
+  version "27.1.4"
+  resolved "https://registry.yarnpkg.com/eslint-plugin-jest/-/eslint-plugin-jest-27.1.4.tgz#bdfaf51ee9ed0b86dac1e83753e22da072acaa92"
+  integrity sha512-evJ9E9id/z2Fu6LR+ncNySJ6UMs5RiJiv4JsmdA3gPWoq0AR+uZyva738+Y9Uln+3WaYX+3OYP9HJoau94Iurg==
+  dependencies:
+    "@typescript-eslint/utils" "^5.10.0"
+
 eslint-plugin-matrix-org@^0.7.0:
   version "0.7.0"
   resolved "https://registry.yarnpkg.com/eslint-plugin-matrix-org/-/eslint-plugin-matrix-org-0.7.0.tgz#4b7456b31e30e7575b62c2aada91915478829f88"