Add needs-run-enabled directive for should-fail tests

I was wary of doing any automatic disabling here, since should-fail
is how we test compiletest itself.

src/test/debuginfo/should-fail.rs

@@ -2,6 +2,7 @@
 
 // == Test [gdb|lldb]-[command|check] are parsed correctly ===
 // should-fail
+// needs-run-enabled
 // compile-flags:-g
 
 // === GDB TESTS ===================================================================================

src/test/ui/meta/revision-bad.rs

@@ -4,6 +4,7 @@
 // run-fail
 // revisions: foo bar
 // should-fail
+// needs-run-enabled
 //[foo] error-pattern:bar
 //[bar] error-pattern:foo