Fix path to test resource files.

The tests are now run from the tests/ subdirectory, not from the
top-level, so adjust the file open paths accordingly.
Include test resource files in the distribution.
Make test resource files available in the build tree.

Use AC_CONFIG_LINKS to create symbolic / hard links / copies so the
tests run from within the build tree can access their data files
stored in the source tree.  This fixes failing tests for VPATH builds.
Signal test failures in the return code.

CU_get_error() only checks for internal CUnit error conditions.
However, the build system expects a non-zero return code in case any
test failed, so we now return the number of failed tests.