diff --git a/composer.json b/composer.json
index 9c8549bea7..347feff8d0 100644
--- a/composer.json
+++ b/composer.json
@@ -81,7 +81,9 @@
     "lint:webp-uploads":           "@lint -- ./plugins/webp-uploads --standard=./plugins/webp-uploads/phpcs.xml.dist",
     "phpstan": "phpstan analyse --memory-limit=2048M",
     "test": "phpunit",
-    "test-multisite": "@test -c tests/multisite.xml",
+    "test-multisite": [
+      "WP_MULTISITE=1 phpunit --exclude-group=ms-excluded"
+    ],
     "test-multisite:plugins": [
       "@test-multisite:auto-sizes",
       "@test-multisite:dominant-color-images",
diff --git a/plugins/dominant-color-images/tests/data/class-testcase.php b/plugins/dominant-color-images/tests/data/class-testcase.php
index 9f39c3111d..966b29f473 100644
--- a/plugins/dominant-color-images/tests/data/class-testcase.php
+++ b/plugins/dominant-color-images/tests/data/class-testcase.php
@@ -175,6 +175,14 @@ public function test_get_dominant_color_valid( string $image_path, array $expect
 	 * @dataProvider provider_get_dominant_color_invalid_images
 	 */
 	public function test_get_dominant_color_invalid( string $image_path ): void {
+		$mime_type = wp_check_filetype( $image_path )['type'];
+		// Old WP does not support ".tiff" and ".bmp" so return false.
+		if ( ! $mime_type ) {
+			$this->markTestSkipped( 'Mime type is not supported.' );
+		}
+		if ( ! wp_image_editor_supports( array( 'mime_type' => $mime_type ) ) ) {
+			$this->markTestSkipped( "Mime type $mime_type is not supported." );
+		}
 		$attachment_id = self::factory()->attachment->create_upload_object( $image_path );
 		wp_maybe_generate_attachment_metadata( get_post( $attachment_id ) );
 
diff --git a/tests/multisite.xml b/tests/multisite.xml
deleted file mode 100644
index 8f03b47fbe..0000000000
--- a/tests/multisite.xml
+++ /dev/null
@@ -1,30 +0,0 @@
-<phpunit
-	bootstrap="./bootstrap.php"
-	backupGlobals="false"
-	colors="true"
-	convertErrorsToExceptions="true"
-	convertNoticesToExceptions="true"
-	convertWarningsToExceptions="true"
-	>
-	<php>
-		<const name="WP_TESTS_MULTISITE" value="1" />
-	</php>
-	<testsuites>
-		<testsuite name="performance-lab">
-			<directory suffix=".php">./</directory>
-			<exclude>./utils</exclude>
-			<exclude>./plugins</exclude>
-		</testsuite>
-		<testsuite name="auto-sizes">
-			<directory suffix=".php">./plugins/auto-sizes</directory>
-		</testsuite>
-		<testsuite name="speculation-rules">
-			<directory suffix=".php">./plugins/speculation-rules</directory>
-		</testsuite>
-	</testsuites>
-	<groups>
-		<exclude>
-			<group>ms-excluded</group>
-		</exclude>
-	</groups>
-</phpunit>