Replace docstrings on test functions witrh comments -- then unittest

prints function and module names, which is more informative now that
we repeat some tests in slightly modified subclasses.

Add a test for read() until EOF.

Add test suites for line-buffered (bufsize==1) and a small custom
buffer size (bufsize==2).

Restructure testUnbufferedRead() somewhat to avoid a potentially
infinite loop.