Migrates remaining continuous integration testing to Github Actions.
Creates multiple GA workflows by platform: ci-x86 (for 32- and 64-bit), ci-aarchxx (for aarchxx cross compile, android arm cross compile, and a64-on-x86), ci-osx (for 64-bit), and ci-clang-format. Multiple smaller workflows are better than a single monolithic one; particularly, this allows re-running a smaller set of jobs instead of all of them (e.g. when re-running due to flaky tests).
Enables GA CI on all push events, which includes commits to all branches, including master.
Adds some comments to previously added test suites too.
This PR also documents some testing done to verify this GA CI setup. Some bugs were added on purpose to various tests, to verify if the corresponding CI(s) fails, which they did.
Issue: #4549 (closed)