Support for unittest subtests

[lrperlmu]

Is your feature request related to a problem? Please describe.
gradescope-utils does not produce output for subtests
(for definition of subtest, see https://docs.python.org/3/library/unittest.html#distinguishing-test-iterations-using-subtests)

Describe the solution you'd like
Output a separate json object for each subtest, so that each subtest appears in the json output just like each test.

Describe alternatives you've considered

• I could avoid using subtests -- I lose some of the power of the unittest framework
• Construct my json output by hand instead of using gradescope-utils

Additional context
gradescope-utils is a nice, useful package, thanks for developing it :)

[lrperlmu]

oops this is a duplicate of #5