Fix fallout from r1537147 and other revision which involved changes to the way the --srcdir option is handled in the C tests.
Originally, this option was used only by config-test.c and was required, ie. the test failed if the option wasn't given (i.e. opt.srcdir == NULL). Since r1537147 these tests would crash if no --srcdir option was given. Some tests in checksum-test.c and repos-test.c have since started using the value of --srcdir, also without checking for NULL.
Add a new helper function svn_test_get_srcdir() for these tests, which falls back to the current working directory if --srcdir isn't given. This allows tests to pass if invoked from within their directory: cd subversion/tests/libsvn_subr && ./config-test
The test runner has always been passing --srcdir to C tests, so the standard 'make check' procedure wasn't affected by this problem.
* subversion/tests/libsvn_repos/repos-test.c (test_config_pool): Use svn_test_get_srcdir().
* subversion/tests/libsvn_subr/checksum-test.c (zlib_expansion_test): Use svn_test_get_srcdir().
* subversion/tests/libsvn_subr/config-test.c (get_config_file_path): New helper function which uses svn_test_get_srcdir() to build the test configuration file's path. (test_text_retrieval, test_boolean_retrieval, test_has_section_case_insensitive, test_has_section_case_sensitive, test_has_option_case_sensitive, test_stream_interface, test_read_only_mode): Use get_config_file_path().