report: fix xunit tests count

report: fix xunit tests count

The xunit "section report" provides a tests attribute, which according
to https://llg.cubic.org/docs/junit/ represents:
tests=""     <!-- The total number of tests in the suite, required. -->

The current value is generated as a sum of the $n_try and $n_notrun
counters. This is incorrect as the $n_try counter already includes tests
which are run but complete with _notrun.
One special case exists for $showme (check -n), where $n_try remains
zero, so $n_notrun can be used as-is.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>