gh-140082: Forward colorizing from libregrtest to unittest (#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.

Co-authored-by: Victor Stinner <vstinner@python.org>