svntest.main: Allow callers to be in places other than tests/cmdline/.

* subversion/tests/cmdline/svntest/main.py
  (P): New helper, limited in scope to about 20 lines of code.
  (svn_binary, svnadmin_binary, svnlook_binary, svnrdump_binary,
   svnsync_binary, svnversion_binary, svndumpfilter_binary, svnmucc_binary,
   entriesdump_binary, lock_helper_binary, atomic_ra_revprop_change_binary,
   wc_lock_tester_binary, wc_incomplete_tester_binary):
     Compute these relative to this file rather than relative to the cwd.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592715 13f79535-47bb-0310-9956-ffa450edef68
svntest.verify.DumpParser: compare property hashes without regard to order of
properties.

Without this, backport_tests.py (to be added, in tools/dist/, in a few commits)
breaks on the revprop comparisons.

* subversion/tests/cmdline/svntest/verify.py
  (DumpParser.get_props):
    Parse the properties hash, return them as a Python dict rather
    than as a str containing a svn_hash_write2(terminator="") serialization.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592717 13f79535-47bb-0310-9956-ffa450edef68
* subversion/tests/cmdline/svntest/sandbox.py
  (Sandbox.youngest): New method.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592718 13f79535-47bb-0310-9956-ffa450edef68