GHA: Avoid multiple test suites per job

Noticed this when looking at GHA results for some on-going work. After JDK-8320921, there is a test job that carries two test suites, and one of them failed:

```
 ==============================
Test summary
==============================
   TEST TOTAL PASS FAIL ERROR
>> jtreg:test/hotspot/jtreg:tier1_compiler_2 271 270 1 0 <<
   jtreg:test/hotspot/jtreg:tier1_compiler_not_xcomp 12 12 0 0
==============================
TEST FAILURE
```

This leads to jtreg correctly creating `exit-with-error`, but overwriting the failure logs from the first suite with the second (passing) suite. This confuses the test results parsing logic. The change we did with JDK-8324659 correctly catches the generic error, which looks for `exit-with-error` in this case:

```
 Error: Unspecified test suite failure. Please see log for job for details.
```

But it is awkward, since it does not parse the test results anymore. Plus, without JDK-8324659, this effectively hides tier1_compiler_2 failures. The easiest way to mitigate this now is avoiding carrying several test suites per job.