Add option to make testrunner ignore knownfailures.json

Sometimes it is useful to make testrunner run a test even if it is
listed in knownfailures.json. This adds a --no-skips option that does
exactly that. If this option is passed testrunner will consider every
variant of every test runnable.

Test: ./test/testrunner/testrunner.py --host --no-skips --redefine-stress -t 974-hiddenapi
Change-Id: Iffc743d515819fa0a3a0f14e2da0bbf2b42673c4