tests: Introduce igt_fixture

tests: Introduce igt_fixture

Just a tiny wrapper to protect global test setup/teardown code when
just listing subtests. Rolling this out over all tests with subtests
should allow us to generate the testlist with piglit as an
unpriviledged user on a non-intel system.

The aim here is to make our QA team happy who currently suffers from
this. Even more so for the prime tests since you need a system with
intel _and_ nouveau gpus to just be able to list tests.

Exemplary conversion with gem_concurrent_blt.c

Fixture is the same name other test suites like googletest use for
setup/teardown code used by multiple tests.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>