An important property for multi-client systems is that each client gets
a 'fair' allotment of system time. (Where fairness is at the whim of the
context properties, such as priorities.) This test forks N independent
clients (albeit they happen to share a single vm), and does an equal
amount of work in client and asserts that they take an equal amount of
time.

Though we have never claimed to have a completely fair scheduler, that
is what is expected.

v2: igt_assert_f and more commentary; exclude vip from client stats,
include range of frame intervals from each individual client
v3: Write down what the test actually does!
v4: Split out to gem_exec_fair so that we can apply CI filtering to
reduce the test set

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>