ci: print stack traces on macOS if any tests fail

ci: print stack traces on macOS if any tests fail

Mostly the tests should fail cleanly, but sometimes the test might crash
or abort. In this case we'll need to know the stack traces in order to
debug the problem. Fortunately on the Cirrus CI macOS instances, the
crash reporting service is active and saving crashes to the directory
~/Library/Logs/DiagnosticReports/

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>