tests/gem_ctx_basic: Use igt_require

tests/gem_ctx_basic: Use igt_require

... instead of threading the 77 return value through threads. Even
better would be to check for requirements in the single-threaded setup
part of the test, since now it's done by multiple threads in parallel
and will result in interleaved and multiplied output on stdout.
Whatever.

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