Update test_commit_txn() to handle svn_fs_commit_txn()'s return semantics.
Update test_commit_txn() to handle svn_fs_commit_txn()'s return
semantics.

* subversion/tests/libsvn_fs/fs-test.c
  (test_commit_txn):
    If svn_fs_commit_txn() returns an error, then always return an
      error to the caller, just use a different wrapping error message
      if the commit succeeded or failed.
    If svn_fs_commit_txn() returns no error, than assert that a valid
      revision number was returned.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion