TEST-23-UNIT-FILE: do not wait indefinitely but set a reasonable timeout

TEST-23-UNIT-FILE: do not wait indefinitely but set a reasonable timeout

Otherwise, the test does not finish until the global timeout is reached.

This is for making the test fail earlier when something spurious happens:
```
[FAILED] Failed to start TEST-23-UNIT-FILE-short-lived.service - Shortlived Unit.
TEST-23-UNIT-FILE.sh[776]: + '[' 0 -eq 0 ']'
TEST-23-UNIT-FILE.sh[776]: + sleep .5
(snip)
58/98 systemd:integration-tests / TEST-23-UNIT-FILE  TIMEOUT  1800.52s  killed by signal 9 SIGKILL
```