Add matrix build for unit tests

[morgo]

Part of #4989

Changes:

• Disable local_example for macOS for now (it is flaky when starting tablets. I looked into it, it's non-obvious. I will fix it by switching the vttablet-up.sh script to be init-style, and have a retry on start.)
• Revert a previous workaround for GitHub actions instability, as it is no longer required Improve Test Stability #5567
• Change checkout action to v2 (faster; only does a shallow checkout)
• Add matrix build for MySQL 5.7 and MySQL 8.0. Plumbing exists for percona 5.6, MariaDB 10.1-10.3, but disabled due to disabled due to Failing Unit Test: vitess.io/vitess/go/mysql with MariaDB 10.2, 10.3 #5569 and Failing Unit Test: vt/vttablet/tabletserver/vstreamer on MySQL 5.6 and MariaDB 10.1 #5571
• Move flaky tests to _flaky.
• Disable go test cache (count=1). It can lead to unreliable results when testing multiple MySQL's locally since modifying environment variables does not purge cache.
• Re-enable unit race, since we now have better handling with flaky tests. Fix unit_race so that it is maintained, but move it to run never.

Signed-off-by: Morgan Tocker [email protected]

[enisoc]

LGTM