test: Fix exceptions in tests (#4772)

This fixes a test exception that did not fail its tests.  An HlsParser
instance outlived the test run and failed after server disconnection.

This issue was spotted while running the tests in a local browser in
debug mode. It caused no test failures directly, but was an example of
poor hygiene in our tests.

test/hls/hls_parser_unit.js

@@ -37,6 +37,7 @@ describe('HlsParser', () => {
 
   afterEach(() => {
     shaka.log.alwaysWarn = originalAlwaysWarn;
+    parser.stop();
   });
 
   beforeEach(() => {