add docs/tests.rst documenting how to run tests and how to interpret the output

[davidsarah]

In ticket:145#comment:26, Zooko wrote about various ways to run the Tahoe-LAFS test suite, some of which work and some of which do not. He suggested adding a docs/tests.rst to document this.

tests.rst should also document cases in which the tests are known to fail (such as when we're not testing the right code), how to interpret skips, todos, and other sources of noise, and how to get Twisted log output from test cases.