[test-runner] Put TestFixture class check to the right place
[test-runner] Put TestFixture class check to the right place

Previously it was done during object instantiation and
was useless.

Change-Id: I10827d2006e11cb734ad436adbf8f56e02710780
Closes-Bug: #1528210

satish-avninetworks/murano,satish-avninetworks/murano,satish-avninetworks/murano,satish-avninetworks/murano,DavidPurcell/murano_temp,NeCTAR-RC/murano,olivierlemasle/murano,olivierlemasle/murano,NeCTAR-RC/murano,openstack/murano,olivierlemasle/murano,DavidPurcell/murano_temp,DavidPurcell/murano_temp,olivierlemasle/murano,NeCTAR-RC/murano,openstack/murano,NeCTAR-RC/murano,DavidPurcell/murano_temp