Add run-make test testing flag stability

src/doc/unstable-book/src/compiler-flags/force-warns.md

@@ -12,7 +12,7 @@ This feature allows you to cause any lint to produce a warning even if the lint
 #![allow(dead_code)]
 
 fn dead_function() {}
-// This would normally not produce a warning even though the 
+// This would normally not produce a warning even though the
 // function is not used, because dead code is being allowed
 
 fn main() {}

src/test/run-make/unstable-flag-required/Makefile

@@ -2,3 +2,4 @@
 
 all:
 	$(RUSTDOC) --output-format=json x.html 2>&1 | diff - output-format-json.stderr
+	$(RUSTC) --force-warns dead_code x.rs 2>&1 | diff - force-warns.stderr

src/test/run-make/unstable-flag-required/force-warns.stderr

@@ -0,0 +1,2 @@
+error: the `-Z unstable-options` flag must also be passed to enable the flag `--force-warns=lints`
+