[examples/systemd activation] multiple fixtures for testing different unix enpoints

[nipil]

The test script only handled one socket type/name at a time. Rewrite the script to handle multiple test fixtures, and add tests easily.

To keep existing behaviour, returns the number of FAILED test as exit value : so when all tests pass, exits with 0, and if any test fails, exit with non-zero.

The tests provided in this PR all pass, when the following PR regarding systemd socket activation fixes are active :

• #36034 (included)
• #36035 (included)
• #36036
• #36039
• #36041

Tested with :

• systemd 247 on debian 11
• iomgr (experiments event_engine_listener posix=false)