ci: integration: Rename all Avocado standard stream log files to *.log

ci: integration: Rename all Avocado standard stream log files to *.log

By default, stdout/stderr Avocado test log files do not have any file
extension which confuses GitLab's web UI to mangle the MIME type for
these and so the browser will never offer the option to open such file
from in a text editor rather than dowloading it.
Since GitLab sets a proper MIME for .txt and .log file extensions,
rename all Avocado log files without an extension to *.log . This pairs
nicely with the coredumpctl info file which we already name as
'coredumpctl.txt' because of this.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>