Improve test flakiness

[tunetheweb]

Makes the following improvements:

• Reverts temp workarounds from Turn off flakey Firefox tests #473, Disable more Firefox tests #475, Better alternative for test flakiness on Firefox for FCP and LCP #476, Facing issue while exporting web-vitals/attribution using browserify #486 as no longer required as discussed in: https://bugzilla.mozilla.org/show_bug.cgi?id=1893921
• Skips prerender on unsupported browser (seemed to be causing flakiness for FCP tests on Safari).
• Adds an extra delay until interactive for the LCP image after hidden test as LCP doesn't always seem to be reported without this in all browsers.
• Adds an extra delay for the FCP initial hidden tests as FCP doesn't always seem to be reported without this in Safari.
• Adds extra config to skip the Search engine choice pop up.

[philipwalton]

I reverted the checks that would skip some tests in browsers that don't support prerender. I don't think the prerender-stubbing code was the cause of the flakiness, I think it was mainly the issue we discovered with the navigateTo() method.

IMO it only makes sense to skip the check if we're actually triggering a prerender load somehow. If we're stubbing it then I think running the tests in all browsers is still useful to check that out assumptions of how things work are interoperable.

[philipwalton]

I've run the tests three times in a row now and they've all passed each time.