Follow-up to r1592987: unbreak backport_tests.py.

With this change, all of the following cases will work:

- run tests with cwd=(subversion/tests/cmdline/ subdir of an out-of-tree build)
- run tests in-tree
- run backport_tests.py

* tools/dist/backport_tests.py
  (chdir): Define earlier in the file.
  (sys.path): Convert the inserted value to abspath.
  (svntest): Change cwd while importing (like win-tests.py does), document
    the reason.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1593014 13f79535-47bb-0310-9956-ffa450edef68