tests: disabled "quota exceeded" test (#2511)

* disabled unreliable test

The "quota exceeded" test is unreliable and failed too often lately for
my taste. Therefore, I'd like to disable it because there is no use in
having such a test.

* corrected PR id in URL

test/tests.bats

@@ -860,6 +860,8 @@ EOF
 }
 
 @test "checking quota: warn message received when quota exceeded" {
+  skip 'disabled as it fails randomly: https://github.com/docker-mailserver/docker-mailserver/pull/2511'
+
   wait_for_changes_to_be_detected_in_container mail
 
   # create user