[3.14] gh-140082: Forward colorizing from libregrtest to unittest (GH-140083) (#140756)

[3.14] gh-140082: Forward colorizing from libregrtest to unittest (GH-140083) (#140756)

gh-140082: Forward colorizing from libregrtest to unittest (GH-140083)

libregrtest redirects test output to a file as part of its operation.
When `unittest` checks to see if it should colorize with
`isatty(sys.stdout)` that fails resulting in no colorizing of the
unittest output.

Update `libregrtest` to set `FORCE_COLOR=1` when redirecting test output
so that unittest will do color printing.
(cherry picked from commit 6ff62ac4fbc3f17529823c146cc3a3f8c21eed42)

Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>