standalone_test: Record the test start time only once

The test output directory is named as the test starting
time, but standalone script gets the start time twice,
this would make the debug directory name different with
its output in debug log.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
standalone: Create a symbolic link for the latest test run

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virttest.version: Get only the first line of top commit cmd output

So we don't get the logs polluted by lines that show
added files.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>