Teach the C tests to recognize a '--help' option.

* subversion/tests/svn_test_main.c
  (help_opt): New option code.
  (cl_options): Add '--help' option. Tweak the whitespace in some other help
    strings.
  (help): New function.
  (main): Recognize and act on the '--help' option.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1465289 13f79535-47bb-0310-9956-ffa450edef68