Merge pull request #386 in CASA/casa6 from CAS-13328 to master
* commit '6bb00eb952e6d125ef70a0bd8ff4ff8175a73660':
Remove restfreqtool from casatasks since it exists in casatestutils. Update tests to point to casatestutils
remove all other helper files only from casa6
Remove the duplicate copies of testhelper.py from casatasks and casatools. Leave the copy of casa5 in place.