Add a new regression test which checks whether error codes are unique.

* build.conf: Add the new test to the build.

* subversion/tests/libsvn_subr: Add 'error-code-test' to svn:ignore.

* subversion/tests/libsvn_subr/error-code-test.c: New.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464448 13f79535-47bb-0310-9956-ffa450edef68