Run tests in parallel

If there are many tests, this gives faster performance on nearly all
modern systems.  If there are few tests, the overhead is minor and
total runtime is trivial anyway.
Slightly better 'gift test' behavior

Running plain 'gift test' all tests with two in parallel at a time.
Running 'gift test @filenames' runs only the tests for those filenames
(without parallelization). Running 'gift test -v' runs all tests in
verbose mode without parallelization, since that breaks verbose mode.

This isn't perfect and the code could use some cleanup, but it's a
start.  I can refactor it later as I acquire more use cases for 'gift
test' command line options.