Refactor some test code to make it easier to extend.
* subversion/tests/libsvn_repos/repos-test.c (nls_receiver_baton, format_segment, nls_receiver, check_location_segments): Add doc strings and constify the expected result pointers. (location_segment_test_t): New struct. (node_location_segments): Move the test data into an array and iterate over it.
Make the unit test for svn_repos_node_location_segments() stronger.
* subversion/tests/libsvn_repos/repos-test.c (strcmp_null): New function. (nls_receiver): Check 'range_end' and 'path' as well as 'range_start'. (node_location_segments): Fix two typos in the expected paths.