Block Test Fixtures: Add additional error message for serialized test…

… fixtures that reserialize identically (#39039)

* Block Test Fixtures: Add additional error message for serialized test fixtures that do not match, but that reserialize identically

* Update error message

test/integration/full-content/full-content.test.js

@@ -203,13 +203,26 @@ describe( 'full post content fixture', () => {
 			try {
 				expect( serializedActual ).toEqual( serializedExpected );
 			} catch ( err ) {
-				throw new Error(
-					format(
-						"File '%s' does not match expected value:\n\n%s",
-						serializedHTMLFileName,
-						err.message
-					)
-				);
+				if (
+					serialize( blocksActual ) ===
+					serialize( parse( serializedExpected ) )
+				) {
+					throw new Error(
+						format(
+							"File '%s' does not match expected value (however, the block re-serializes identically so you may need to run 'npm run fixtures:regenerate'):\n\n%s",
+							serializedHTMLFileName,
+							err.message
+						)
+					);
+				} else {
+					throw new Error(
+						format(
+							"File '%s' does not match expected value:\n\n%s",
+							serializedHTMLFileName,
+							err.message
+						)
+					);
+				}
 			}
 		} );
 	} );