Document  how to run the test suite

Points to improving the README in  `irods/test`:
   - Describe preconditions for test run (e.g. should be run on - within explicitly stated limits- a fresh iRODS install)
   - Provide usage examples: command line for running test suite or individual tests, or test suite subgroups.
   - Should it be made into a .md (markdown) file, to be consistent with the main README