Temporarily add a dummy test file

For test file that was removed in roast master but is still present in 6.c-errata.

This file will go away once we teach Rakudo's make spectest to use different lists
of test files for each branch of of roast

S15-nfg/grapheme-break.t

@@ -0,0 +1,6 @@
+use v6;
+use Test;
+plan 1;
+# https://github.com/rakudo/rakudo/commit/7f970357bd59c011e9f998ba885d317401851781
+ok 1, 'dummy test file';
+