The Xunit and JSON test cases inherit from PluginsTest. This test defines
some hardcoded-values tests, which are then inherited and run for each
inherited class with the exact same values. This patch creates abstract
class, which all 3 cases share and makes them inherit from it, rather
than from the actual testcase.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>