plugins: don't fail on missing plugin files #850

Job	Run time
check-code (ubuntu-latest, 8.2)	2m 0s
Get PHP versions to test	2s
check-translation (ubuntu-latest, 7.2)	20s
test (ubuntu-latest, 8.0)	1m 2s
test (ubuntu-latest, 8.1)	44s
test (ubuntu-latest, 8.2)	51s
test (ubuntu-latest, 8.3)	42s
test (ubuntu-latest, 7.0)	1m 21s
test (ubuntu-latest, 7.1)	52s
test (ubuntu-latest, 7.2)	46s
test (ubuntu-latest, 7.3)	1m 5s
test (ubuntu-latest, 7.4)	1m 7s
test (ubuntu-latest, 5.6)	1m 0s
	11m 52s

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