[3.12] gh-111881: Import doctest lazily in libregrtest (GH-111884) (#111893)

gh-111881: Import doctest lazily in libregrtest (GH-111884)

In most cases, doctest is not needed. So don't always import it at
startup. The change reduces the number of modules already
imported when a test is run.
(cherry picked from commit 6f09f69b7f85962f66d10637c3325bbb2b2d9853)

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