report: clarify the meaning of the timestamp attribute

report: clarify the meaning of the timestamp attribute

We've never specified what the timestamp attribute of the testsuite
element actually means, and it history is rather murky.

Prior to the introduction of the xml report format in commit f9fde7db2f,
the "date_time" variable was used only to scrape dmesg via the /dev/kmsg
device after each test.  If /dev/kmsg was not a writable path, the
variable was not set at all.  In this case, the report timestamp would
be blank.

In commit ffdecf7498a1, Ted changed the xunit report code to handle
empty date_time values by setting date_time to the time of report
generation.  This change was done to handle the case where no tests are
run at all.  However, it did not change the behavior that date_time is
not set if /dev/kmsg is not writable.

Clear up all this confusion by defining the timestamp attribute to
reflect the start time of the most recent test, regardless of the state
of /dev/kmsg.  If no tests are run, then define the attribute to be the
time of report generation.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>