Fix some portability problems with the python tests for obliterate which I

Fix some portability problems with the python tests for obliterate which I
added in r889342. Remove the "import csvn" statement, as that module may not
be available and is not needed. Ignore any error in trying to dump the BDB
repository state using an external "db_dump" program, as that program may not
be available and the purpose is only to assist with external debugging.
When creating the empty repository, use a function that deletes any previous
repository at that location, so that it does not fail if a previous test run
did not clean up.

* subversion/tests/cmdline/svntest/objects.py
  Remove the "import csvn" statement and the associated commented-out usage.

* subversion/tests/cmdline/svntest/actions.py
  (guarantee_empty_repository): New function.

* subversion/tests/cmdline/obliterate_tests.py
  (obliterate_1): Use guarantee_empty_repository() instead of trying to
    create a repository directly. If an exception is thrown when dumping the
    BDB repository state, ignore it and continue.

git-svn-id: http://svn.apache.org/repos/asf/subversion/trunk/subversion/tests/cmdline@889568 13f79535-47bb-0310-9956-ffa450edef68