plainbox:units:testplan: fix get_effective_category

This patch fixes TestPlan.get_effective_category() when no overrides
are being applied and the natural category is supposed to be returned.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/test-plan-fixes/ by tarmac [r=roadmr][bug=][author=zkrynicki]"