(fix): if custom jest config path, use its dir as rootDir

- that matches Jest's default and means no changes have to be made for
  the internal jest.config.json to work with `tsdx test`

src/index.ts

@@ -502,7 +502,7 @@ prog
     let jestConfig = {
       ...createJestConfig(
         relativePath => path.resolve(__dirname, '..', relativePath),
-        paths.appRoot
+        opts.config ? path.dirname(opts.config) : paths.appRoot
       ),
       ...appPackageJson.jest,
     };

test/jest.config.json

@@ -1,6 +1,5 @@
 {
   "testEnvironment": "node",
-  "rootDir": "./test",
   "roots": ["<rootDir>/tests"],
   "collectCoverageFrom": ["**/*.js"],
   "transform": {