These updates support running tests that have been relocated into `serial` and `parallel/set*` directories.
- `make tests` now calls the two make targets beneath it. The only difference is that `serial` continues the "1 test at a time" approach used prior to this PR, while the `parallel` target increases the `--jobs` arg to run multiple tests concurrently (_configured by `PARALLEL_JOBS`_).
- The `test/%` target leverages Bash syntax magic to ease running single tests without providing the exact path.
- This syntax also supports providing multiple test names (eg: `make test/clamav,template`) to run.
- `**` (globstar) allows for future improvements that can group multiple test files into sub-directories by their scope (eg: anti-spam, ssl, etc).
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chore: Add `shopt -s globstar` to other targets
I realized that other targets should have this as well in case it is not set.
It is better to be more explicit here than to have weird errors due to `**` not expanding properly.
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fix(Makefile): Add back `.PHONY` targets
I encountered `make` telling me the target was already up-to-date, which of course is nonsense.
I therefore added back the `.PHONY` targets to ensure tests are always run.
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docs: Added instructions for running a single test
See https://github.com/docker-mailserver/docker-mailserver/pull/2857/files#r1008582760
