tests(CI): Adjust Makefile & GHA workflow to support new test layout

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).

---

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.

---

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. 

---

docs: Added instructions for running  a single test
See https://github.com/docker-mailserver/docker-mailserver/pull/2857/files#r1008582760

.github/workflows/generic_test.yml

@@ -14,6 +14,9 @@ jobs:
   run-tests:
     name: 'Test'
     runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        part: [serial, parallel/set1, parallel/set2, parallel/set3]
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -45,6 +48,6 @@ jobs:
           cache-from: type=local,src=/tmp/.buildx-cache
 
       - name: 'Run tests'
-        run: make generate-accounts tests
+        run: make generate-accounts tests/${{ matrix.part }}
         env:
           CI: true