gs-plugin-loader: Rework plugin setup logging

This reduces code duplication, and downgrades the warning to a debug
statement if running in the tests, as CI machine slowness can cause
spurious failures otherwise (warnings are fatal in the tests).

Signed-off-by: Philip Withnall <withnall@endlessm.com>
5 jobs for sysprof-4-followup in 10 minutes and 46 seconds
Status Job ID Name Coverage
  Build
passed #802570
debian-stable-x86_64

00:01:38

failed #802571
fedora-x86_64

00:10:05

failed #802549
debian-stable-x86_64

00:02:21

failed #802546
debian-stable-x86_64

00:01:39

failed #802545
fedora-x86_64

00:11:22

 
Name Stage Failure
failed
fedora-x86_64 Build
Uploading artifacts...
_build/config.h: found 1 matching files

_build/meson-logs: found 4 matching files

_build/fedora-x86_64-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=802571 responseStatus=201 Created token=1S65cCWb
Uploading artifacts...
_build/fedora-x86_64-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=802571 responseStatus=201 Created token=1S65cCWb
ERROR: Job failed: exit code 1