Truncate stdout and stderr in the way that it hopefully shows all
important information. At least 15 lines of stdout (Ansible tasks) and
remaining from stderr to fill up to maxlen size.

The test log mostly only showed lines from stderr as the log was a
concatenation of stdout and stderr and only the last 2500 chars of this
have been used. This was hiding the test result from stdout and failures
have not beeen visible.