Configure phpunit-watcher to improve devex. (#27058)

This provides similar features for PHP tests that Jest provides for JavaScript tests.

The `process-timeout` option is disabled so that phpunit-watcher can run indefinitely. See spatie/phpunit-watcher#63 (comment)".

.gitignore

@@ -23,3 +23,5 @@ test/native/junit.xml
 
 # Local overrides
 .wp-env.override.json
+phpunit.xml
+phpunit-watcher.yml

composer.json

@@ -10,19 +10,26 @@
 	"support": {
 		"issues": "https://github.com/WordPress/gutenberg/issues"
 	},
+	"config": {
+		"process-timeout": 0
+	},
 	"require-dev": {
 		"dealerdirect/phpcodesniffer-composer-installer": "^0.7",
 		"squizlabs/php_codesniffer": "^3.5",
 		"phpcompatibility/php-compatibility": "^9.3",
 		"wp-coding-standards/wpcs": "^2.2",
 		"sirbrillig/phpcs-variable-analysis": "^2.8",
-		"wp-phpunit/wp-phpunit": "^5.4"
+		"wp-phpunit/wp-phpunit": "^5.4",
+		"phpunit/phpunit": "^7.5.20",
+		"spatie/phpunit-watcher": "^1.23"
 	},
 	"require": {
 		"composer/installers": "~1.0"
 	},
 	"scripts": {
 		"format": "phpcbf --standard=phpcs.xml.dist --report-summary --report-source",
-		"lint": "phpcs --standard=phpcs.xml.dist --runtime-set ignore_warnings_on_exit 1"
+		"lint": "phpcs --standard=phpcs.xml.dist --runtime-set ignore_warnings_on_exit 1",
+		"test": "phpunit",
+		"test:watch": "phpunit-watcher watch < /dev/tty"
 	}
 }