Minimize stdout logging in pairwise testing jobs

The size of the log generated by each GitLab CI job is limited to 4 MB by default. While this limit is configurable, it makes little sense to print build logs to standard output if they are being captured to files anyway. Limit use of "tee" in util/pairwise-testing.sh to printing the combination of configure switches used for a given build. This way the job should never exceed the default 4 MB log size limit, yet it will still indicate its progress in a concise way.