tests: fix a timestamp race in python tests

tests: fix a timestamp race in python tests

Fixes automake bug#12210.

* t/python-missing.sh: Call aclocal and autoconf with the "--force"
option.  We need this because, on fast machines, it's possible for
'mypy.m4' and 'aclocal.m4' to end up with the same timestamp as configure,
so autoconf (without the "--force" options) wouldn't bother to rebuild it,
and would just rerun the previous AM_PATH_PYTHON test, succeeding rather
than failing as expected.
* t/python-am-path-iftrue.sh: Likewise.

Co-authored-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Copyright-paperwork-exempt: yes
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>