patch 9.1.0841: tests: still preferring python2 over python3

patch 9.1.0841: tests: still preferring python2 over python3

Problem:  tests: still preferring python2 over python3
Solution: prefer Python 3 when picking a Python program in Vim tests,
          by checking for the more specific python version first and
          only when python3 not found, check for the python binary
          (Yee Cheng Chin)

Most OSes have Python 3 mapped to `python3` instead of `python`. Vim
tests should prioritize using that instead of Python 2 in case that is
still installed on the host system.

closes: #15986

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>