t: fix out-of-tree tests for some git-p4 tests

t: fix out-of-tree tests for some git-p4 tests

Both t9835 and t9836 exercise git-p4, but one exercises Python 2 whereas
the other one uses Python 3. These tests do not exercise "git p4", but
instead they use "git p4.py". This calls the unbuilt version of
"git-p4.py" that still has the "#!/usr/bin/env python" shebang, which
allows the test to modify which Python version comes first in $PATH,
making it possible to force a Python version.

But "git-p4.py" is not in our PATH during out-of-tree builds, and thus
we cannot locate "git-p4.py". The tests thus break with CMake and Meson.

Fix this by instead manually setting up script wrappers that invoke the
respective Python interpreter directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>