gramineproject
[tests] Fix tampering tests for protected files
Description of the changes
Test test_500_invalid always failed as test files to decrypt always had an illegal filename masking the actual intended tampering. Fixing this in libos/test/fs/test_enc.py showed that a number of intended tampering use-cases in fact were uncatchable. This PR fixes the file path in the test driver test_nec.py, removes ineffective tampering use-cases and adds some tampering with ciphertext on headers.
The PR also (in a separate commit) improves debuggability of python tests by optionally allow to skip the teardown of the test artifacts and run all tests for 500 instead of aborting on first failure.
How to test this PR?
Run usual regression tests
This change is 
@kailun-qin somehow my force-push (with just docu update) in response to Michal's review invalided your approval (but interestingly not Dmitriis). Hence i did re-request another review. Sorry for the hassle ...