Add test coverage job

[iluuu1994]

Coverage was previously executed in nightly and honestly I don't think most people even knew it was there. It takes a bit longer than the other jobs but it's a useful metric and might as well be more visible.

Targeting PHP 8.0 not because it's super useful there but to avoid conflicts when merging other CI changes upstream.

TODO:

• Since we already have a --enable-debug --disable-zts job we probably might as well just combine the two, unless the job takes a significantly longer amount of time (I'll check after the jobs have finished)
• Does the coverage change significantly with only the OpCache tests? It not, skip non-OpCache tests
• Ignore coverage for zpp and other undesired macros We can do that on master

[cmb69]

Thank you for working on this! I don't have a strong opinion about running for each push or nightly only, but one problem I see with coverage reports is that they may encourage users to provide potentially useless or even harmful tests for the sake of increased coverage. We should probably add some info about that to https://github.com/php/php-src/blob/master/CONTRIBUTING.md#writing-tests.

[iluuu1994]

Maybe it's possible to ignore certain lines like ZPP to avoid encouraging useless tests, like adding LCOV_EXCL_START/STOP to the given macros.

https://stackoverflow.com/a/23264375/1320374

[iluuu1994]

I changed my mind and moved this to nightly. It's also probably enough to do this for master, coverage of support branches shouldn't change much.