tests: Decrease the iterations of the checkpoint test to 3.

Currently, the test in realview64-simple-atomic-checkpoint.py runs the
default number of checkpoint cycles which is 5. Each of these cycles
takes a long time to run (approximately 5 minutes) which makes this test
take a long time to run to completion. We would get almost all of the
benefit of this test in a fraction of the time if we reduce the number
of iterations down to 3. This still has enough iterations to catch bugs
which would happen in one, a different class of bugs which happen when
checkpointing more than once, and even a third iteration for safety.
Because this test is one of the (if not the) most time consuming test in
the quick tests, reducing its length will significantly improve turn
around time when running all those tests.

That is especially valuable when running the tests multiple times to try
to iterate on a bug, or when sweeping through a series of changes trying
to identify the source of breakages. In cases where we might need to
identify failures which don't effect the end product of a series, ie where
a problem is introduced and then coincidentally fixed in a later change,
this is especially important since every individual change needs to be
checked, not just log(n) of the changes like in a binary search.

Ideally in the future we will also try to figure out why taking and
restoring from checkpoints takes so long so that each iteration of this
test is substantially less expensive. If we do that, we could consider
increasing the iteration count again.

Change-Id: Icfa2a391a4a7457d893f2063fab231d22e572deb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53123
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

tests/gem5/configs/realview64-simple-atomic-checkpoint.py

@@ -43,5 +43,5 @@ root = LinuxArmFSSystemUniprocessor(mem_mode='atomic',
                                     mem_class=SimpleMemory,
                                     cpu_class=AtomicSimpleCPU).create_root()
 
-run_test = functools.partial(checkpoint.run_test, interval=0.2)
-
+run_test = functools.partial(checkpoint.run_test, interval=0.2,
+                             max_checkpoints=3)