dma-buf/selftests: drop the mock_wait implementation

dma-buf/selftests: drop the mock_wait implementation

Actually test the documented expectation of dma_fence_wait() instead of
comming up with a mock_wait implementation which uses undocumented
and non standard return codes and behavior.

Additional to that increase the timeout to one second, otherwise the
test case is a bit unreliable.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://lore.kernel.org/r/346e3df8-5f3a-4394-9d90-4ba107294161@ursulin.net