Add a basic test for svn_fs_merge.  The interesting, complicated cases are
being tested through the respective commit test cases.

* subversion/tests/libsvn_fs/fs-test.c
  (test_fs_merge): New test case.
  (test_funcs): Register new test case.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1637400 13f79535-47bb-0310-9956-ffa450edef68