svntest: Usability fix for manual (selective) runs.

* subversion/tests/cmdline/svntest/main.py
  (execute_tests): Strip trailing commas from the "test function name" argument
    before matching it with the known function names.  That will allow me to
    copy-paste test names into the shell even if my terminal's "word selection"
    includes the comma in the highlighted-by-double-click range.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1512412 13f79535-47bb-0310-9956-ffa450edef68