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().

* subversion/tests/svn_test.h
  (svn_test_get_srcdir): Declare.

* subversion/tests/svn_test_main.c
  (svn_test_get_srcdir): New helper function to obtain the value of
   the --srcdir option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603816 13f79535-47bb-0310-9956-ffa450edef68