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>
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passed #802593
debian-stable-x86_64

00:02:34

passed #802592
fedora-x86_64

00:04:58