common.bash: run `cmd` only if a test case was found

It's only useful to run `cmd` in `for_each_unittest` if a test case
was found. This change allows us to remove the guards from the
functions `run_task` and `mkstandalone`.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200923134758.19354-2-mhartmay@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>