Document how to run the test suite

[d-w-moore]

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

[d-w-moore]

A lot of the howto is now is implemented / codified in the new testing methodologies of PR #772. There's also ample README under the test_harness directory in that same PR. Should we close this issue?

[alanking]

Yes, let's do it. Good catch.