ext: Add timing indications to every TestCase

ext: Add timing indications to every TestCase

The log_call helper is now accepting a time parameter (dictionary). If
the param is not None, the function will fill the timing indications
(user and system time) for the TestCase.

There are some TestCases whose user time is not of our interest; for
example we don't really care about the cpu time of a stdout diff
(MatchStdout tests). In those cases the resulting cpu time in the
generated JUnit file (results.xml) will be 0.

JIRA: https://gem5.atlassian.net/browse/GEM5-548

Change-Id: I53c1b59f8ad93900aeac06197e39189c00a9053c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32653
Tested-by: kokoro <noreply+kokoro@google.com>