test: add empty unittests.js for TypeScriptSupport extension

The test runner loads every default extension's unittests.js. TypeScriptSupport
had none, so the request 404'd and the returned HTML error page was parsed as JS
("SyntaxError: Unexpected token '<'"), reported as a suite-load failure. Add an
empty unittests.js, matching the convention used by other extensions that are
loaded in test windows but ship no unit tests (LightTheme, DarkTheme,
HandlebarsSupport). Git avoids this only because it is excluded from test windows.

Author: abose