Add perflab_disable_object_cache_dropin filter

[tillkruss]

Summary

Fixes #628 and #629.

Relevant technical choices

Using a filter is easy for other plugin to use, while the existing constant would need to be set in the wp-config.php which isn't easily controlled by a plugin.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[felixarntz]

@tillkruss Two minor points of feedback here, also can you please add a test covering the filter?

E.g. like this:

	public function test_perflab_maybe_set_object_cache_dropin_disabled_via_filter() {
		global $wp_filesystem;

		$this->set_up_mock_filesystem();

		// Ensure PL object-cache.php drop-in is not present and constant is not set.
		$this->assertFalse( $wp_filesystem->exists( WP_CONTENT_DIR . '/object-cache.php' ) );
		$this->assertFalse( PERFLAB_OBJECT_CACHE_DROPIN_VERSION );

		// Add filter to disable drop-in.
		add_filter( 'perflab_disable_object_cache_dropin', '__return_true' );

		// Run function to place drop-in and ensure it still doesn't exist afterwards.
		perflab_maybe_set_object_cache_dropin();
		$this->assertFalse( $wp_filesystem->exists( WP_CONTENT_DIR . '/object-cache.php' ) );
	}

[felixarntz]

Thanks @tillkruss!

[mukeshpanchal27]

Thanks @tillkruss, LGTM!