HTML Reporter: Fix display: none regression on global error message

Follows-up 8f5e7ed, which fixed the bug where in the "No tests" error
(or indeed any global error after tests are done running) could cause
the test runner to get stuck.

That change maintained backward compatibility by continuing to count
these global errors as "failed tests", so that CI reporters listening
for `QUnit.done()` on `QUnit.on("runEnd")` continue to get the same
negative signal. And, it continued to visually render them as a failed
test if they happen during test execution or as the last/only event.

But... it inserted the element into the DOM without making visible,
which fooled our internal unit tests.

Fix #1651.

src/html-reporter/html.js

@@ -1054,6 +1054,9 @@ export function escapeText( s ) {
 		assertLi.className = "fail";
 		assertLi.innerHTML = message;
 		assertList.appendChild( assertLi );
+
+		// Make it visible
+		testItem.className = "fail";
 	} );
 
 	// Avoid readyState issue with phantomjs