Improve the test suite's detection of failure to run a test. When a Python

Improve the test suite's detection of failure to run a test. When a Python
test program throws an exception in trying to run a test, make it return
exit code 2 instead of 1. This helps to fulfil the assumption made in
build/run_tests.py that the test program printed its own failure message if
its exit code was 1.

* subversion/tests/cmdline/svntest/main.py
  (run, run_one_test, _internal_run_tests): Document the return value and the
    conditions in which it exits the program.
  (run_tests_body): New function, from the body of run_tests(), but returning
    an exit code rather than terminating with sys.exit().
  (run_tests): Become a wrapper around run_tests_body(), and catch exceptions
    and exit with code 2 in that case.

git-svn-id: http://svn.apache.org/repos/asf/subversion/trunk/subversion/tests/cmdline@880057 13f79535-47bb-0310-9956-ffa450edef68