t983*: use prereq to check for Python-specific git-p4(1) support

t983*: use prereq to check for Python-specific git-p4(1) support

The tests in t9835 and t9836 verify that git-p4(1) works with both
Python 2 and 3, respectively. To determine whether we have those Python
versions in the first place we create a wrapper script that directly
executes the git-p4(1) script with `python2` or `python3` binaries. We
then condition the execution of tests on whether that wrapper script can
be executed successfully.

The logic that does all of this is not contained in a prerequisite block
though, so the output it generates causes us to break the TAP format.
Refactor the logic to use `test_lazy_prereq()` to fix this.

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