[PATCH] fixed python executable for svntest

As part of the Python 2->3 migration effort in Fedora we've been trying
to get to the point where setting PYTHON=/usr/bin/python3 allows us to
ensure that /usr/bin/python is never used in the build environment.

One thing that doesn't work right with svn is that the test scripts are
run by executing shebang scripts with "#!/usr/bin/env python", which
means the PYTHON variable set in make is ignored. This is trivial to fix
by adopting the same logic as used for Win32 already.

Are there any objections to doing this?

[[[
* subversion/tests/cmdline/svntest/main.py
  (open_pipe): Run Python tests under the same Python executable for
  all platforms rather than just Win32 (rather than finding one from
  $PATH via /usr/bin/env).
]]]