This moves us further in the direction of using normal unittest facilities
instead of specialized regrtest ones.  Any test module that can be correctly
run currently using 'python unittest -m test.test_xxx' can now be converted to
use normal unittest test loading by simply deleting its test_main, thus no
longer requiring manual maintenance of the list of tests to run.  (Not all
tests can be converted that easily, since test_main sometimes does some
additional things (such as reap_children or reap_threads).  In those cases the
extra code may be moved to setUpModule/tearDownModule methods, or perhaps the
same ends can be achieved in a different way, such as moving the decorators to
the test classes that need them, etc.)

I don't advocate going through and making this change wholesale, but any time
a list of tests in test_main would otherwise need to be updated, consideration
should instead be given to deleting test_main.