GoogleTest: Add working directory to gtest_discover_tests error message

If the working directory didn't exist, the old message just says:

    Error running test executable.

        Path: '<Path to test executable>'
        Result: No such file or directory
        Output:

This leads the user to the conclusion that the test executable doesn't
exist, which isn't true.  Make the true cause visible by reporting the
working directory in the error message.
3 files changed