Implementing a happy idea from Georg Brandl: make runtest() try to

Implementing a happy idea from Georg Brandl:  make runtest() try to
clean up files and directories the tests often leave behind by
mistake.  This is the first time in history I don't have a bogus
"db_home" directory after running the tests ;-)

Also worked on runtest's docstring, to say something about all the
arguments, and to document the non-obvious return values.

New functions runtest_inner() and cleanup_test_droppings() in
support of the above.