Replace usage of `runkit7_constant_remove` in tests

[zutigrm]

Feature Description

We have several tests that use runkit7_constant_remove to remove declared variables in tests. We should refactor this approach to use @runInSeparateProcess like we do in several other tests, for example here:

site-kit-wp/tests/phpunit/integration/Core/Authentication/Google_ProxyTest.php

Lines 168 to 179 in 0c1cc33

	/**
	* @runInSeparateProcess
	*/
	public function test_url_handles_staging() {
	$url = $this->google_proxy->url();
	$this->assertEquals( $url, Google_Proxy::PRODUCTION_BASE_URL, 'URL should be production base URL by default.' );
	define( 'GOOGLESITEKIT_PROXY_URL', Google_Proxy::STAGING_BASE_URL );
	$url = $this->google_proxy->url();
	$this->assertEquals( $url, Google_Proxy::STAGING_BASE_URL, 'URL should be staging base URL when constant is set.' );
	}

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Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• All test usage of runkit-related functions (e.g. runkit7_constant_remove) is refactored to use @runInSeparateProcess instead.
  • Declaring constants in test classes should be done using the standard @runInSeparateProcess approach (as seen in tests like Google_ProxyTest).
• The runkit and runkit7 extensions are removed from the PHP Test workflow configuration.

Implementation Brief

• Replace every test-time constant reset that currently relies on runkit:
  • For each affected test class (WP_COM_CheckTest, the conversion-event provider tests for WPForms, Ninja Forms, Mailchimp, OptinMonster, Contact Form 7, Popup Maker), remove the tear_down_after_class method that calls runkit*_constant_remove.
  • Add @runInSeparateProcess to every test method that defines a plugin/host constant (define( 'WPFORMS_VERSION', … ), etc.) so the definition occurs in its own PHP process and does not leak to other tests.
  • Remove the existing skip guards (if ( defined( … ) ) $this->markTestSkipped) in WP_COM_CheckTest.
• In .github/workflows/php-lint-tests.yml, remove runkit7 and runkit from the extensions list for the PHPUnit jobs.

Test Coverage

• No changes, issue about test edits

QA Brief

Changelog entry