add regression coverage for buildtest report clear

[shahzebsiddiqui]

see https://app.codecov.io/gh/buildtesters/buildtest/blob/devel/buildtest/cli/report.py

I suppose since we need the report files we should consider creating backups of file during test just so we can remove it and then copy the files back since other test may rely on the report and having it removed can cause some tests to fail.