MFC several WITH_TESTS-related documentation fixes.

- r258014 Document WITH_TESTS src.conf(5) option.
- r263089 Fix description of WITHOUT_BMAKE's purpose.
- r264408 Fix test suite name in src.conf(5).
- r264410 Document how to install the test suite.
- r264412 Add FreeBSD 10.1 to the list of recognized releases for Fx.
- r264413 Miscellaneous fixes to tests(7).