testsuite: Introduce fragile modifier
Now since we have been a bit more stringent in testsuite cleanliness we
have been marking a lot of tests as fragile using the skip modifier.
However, this unfortunately means that we lose the association with the
ticket number documenting the fragility.
Here we introduce fragile and fragile_for to retain this
(cherry picked from commit 4ca271d1880a6f4c5f49869de7f1920a2073adb6)