Fix AMU non-zero counters test

The condition checked in one of the AMU tests (AMU counters always
non-zero) does not always hold. The counter that counts the memory stall
cycles, under certain circumstances, can be zero. Hence, the test is
adjusted accordingly and, consequently, renamed.

Change-Id: I8c7300481c2b45825101ee87f61c68c2ab51758a
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>
2 files changed