gem_concurrent_blit tries to ensure that it doesn't try and run a test
that would grind the system to a halt, i.e. unexpectedly cause swap
thrashing. It currently calls intel_require_memory(), but outside of
the subtest (as the tests use fork, it cannot do requirement testing
within the test children) - but intel_require_memory() calls
igt_require() and triggers and abort. Wrapping that initial require
within an igt_fixture() stops the abort(), but also prevents any further
testing.

This patch restructures the requirement checking to ordinary conditions,
which though allowing the test to run, also prevents listing of subtests
on machines which cannot handle them.