Monorepo: Restore and add test coverage config (#39961)

* Enable and normalize test coverage commands

* Revert "Code Coverage: remove automation (#28542)"

This reverts commit da5e78803f56ea0fe54f29720c05be037093b97a.

* Revert "Remove remnants of automated code coverage support (#28982)"

This reverts commit 8cc32461b2d3b7e3a4a391c033163e5ebc3822fa.

* Fix starter plugin test broken by #31417

* Plugin tests are now run in the monorepo rather than wp-content/plugins.

* Removed undocumented WP_DEVELOP_DIR variable, documented WORDPRESS_DEVELOP_DIR and WORDPRESS_DIR

* Fixed JS tests in some packages where .scripts.test-js had never been added to composer.json.

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Co-authored-by: Brad Jorsch <brad.jorsch@automattic.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/11610578911

Upstream-Ref: Automattic/jetpack@7fadf4a

composer.json

@@ -5,7 +5,7 @@
 	"license": "GPL-2.0-or-later",
 	"require": {},
 	"require-dev": {
-		"automattic/jetpack-changelogger": "^4.2.7"
+		"automattic/jetpack-changelogger": "^4.2.8-alpha"
 	},
 	"scripts": {
 		"build-development": "./tools/build",