allow reuse of _source_submits_a_file method in other functional tests
assert that a submitted secret message cannot be found in a memory dump of the source server
adds test case for message submission not being retained in memory
adds gdb dependency to vagrant, needed by new memory test
makes _source_submits_a_message function reusable
supressed gcore output in tests
make tests fail with the number of occurences of the secret_message in memory
Merge pull request #476 from jacksingleton/submission-in-mem-test

Test to check if the source submission is retained in memory