In the Windows build and tests, be more robust when invoking Python scripts

In the Windows build and tests, be more robust when invoking Python scripts
by using the full path to the Python interpreter.  (This avoids assuming
that 'python foo.py' or 'foo.py' will run the script.)

Thanks to Bert and gstein.

* build/generator/svn_config.vcproj.ezt
  (sql):  Add the Python interpreter's path to the command.

* build/generator/gen_vcnet_vcproj.py
  (Generator.write):  Pass the interpreter's pass to the template.

* subversion/tests/cmdline/svneditor.bat:
    Run svneditor.py using the python in %SVN_TEST_PYTHON%.

* subversion/tests/cmdline/svntest/main.py
  (use_editor):
    Set SVN_TEST_PYTHON.

git-svn-id: http://svn.apache.org/repos/asf/subversion/trunk/subversion/tests/cmdline@879963 13f79535-47bb-0310-9956-ffa450edef68