op-test: refactor conditional import of xmlrunner
op-test: refactor conditional import of xmlrunner

Rather than having two similar construtor/run paths, this change just
sets the runner class and arguments, and constructs & runs the tests in
common code.

Signed-off-by: Jeremy Kerr <68a658b44bbaefab3ef5cd231967195fc0ff9d69@ozlabs.org>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework