The attached diff adds a "-o" ("--one") option to the "unittest" module
that causes the run to abort on the first error encountered.  I name my
tests so that the lowest-level tests get run first so stopping at the
first error tends to prevent a lot of dependent errors and speed
debugging.  During development, I typically run the tests I'm writing
with "-ov".  During a full test run, I omit both those flags.