Re-enable browser tests

[jsantell]

Previously, we were using @web/test-runner for the browser-specific tests in common-iframe-sandbox and common-identity to test iframe CSP and WebCrypto browser compat fallbacks specifically. These are important tests to have, and were temporarily disabled in the denoification #477. Currently, @web/test-runner isn't working "out of the box" and some additional configurations, or an alternative, is needed.

[jsantell]

Fixed in #501