C tests harness: print symbolic error names upon FAIL.

Example output:

    subversion/tests/libsvn_fs/fs-test.c:4975: (apr_err=SVN_ERR_TEST_FAILED)
    svn_tests: E200006: Expected error SVN_ERR_FS_CORRUPT but got SVN_ERR_FS_PATH_SYNTAX
    subversion/libsvn_fs/fs-loader.c:1166,
    subversion/libsvn_fs_fs/tree.c:2508,
    subversion/libsvn_fs_fs/tree.c:2234: (apr_err=SVN_ERR_FS_PATH_SYNTAX)
    svn_tests: E160005: Invalid control character '0x0a' in path '/bar\012'
    FAIL:  lt-fs-test 38: filenames with trailing \n might be rejected
    zsh: exit 1     ../runctest fs fs 38

* subversion/tests/svn_test.h
  (SVN_TEST_ASSERT_ERROR): Print symbolic, not numeric, error codes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481214 13f79535-47bb-0310-9956-ffa450edef68