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>

lib/drmtest.h

@@ -174,6 +174,8 @@ void igt_exit(void) __attribute__((noreturn));
  */
 #define igt_require(expr) do { if (!(expr)) __igt_skip_check(__FILE__, __LINE__, __func__, #expr ); } while (0)
 
+#define igt_fixture if (!igt_only_list_subtests())
+
 /* check functions which auto-skip tests by calling igt_skip() */
 void gem_require_caching(int fd);
 static inline void gem_require_ring(int fd, int ring_id)