Add decorators for functional and performance test cases. These
decorators add attributes to the decorated test cases.

With the addition of decorators, we change the test case discovery
mechanism from looking at test case names according to a regex to simply
checking an attribute of the function added with one of the decorators.

The decorators allow us to add further variables to test cases.

Also move the test case filtering to TestSuite while changing the
mechanism to separate the logic in a more sensible manner.

Bugzilla ID: 1460

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>