Merge pull request #222 in CASA/casa6 from CAS-12987 to master
* commit '673e7875cc5bc2a996d7ef06fd8ec6965cd38ad3':
updated tests for various casa5/6 changes in behavior upon task failure
move old imhead tests to new req tests. remove old imhead test files
added tests from old test_imhead.py
updated imhead tests
more test_req_task_imhead.py mods
updated imhead tests
add testing of unsupported mode
made code easier to read