xfail some tests that hang (instead of segfaulting) since eafb6789a2

See issue #936

src/test/compile-fail/extfmt-non-literal2.rs

@@ -1,5 +1,8 @@
 // error-pattern: literal
 
+// xfail-test (issue #936)
+
+
 fn main() {
     // #fmt's first argument must be a literal.  Hopefully this
     // restriction can be eased eventually to just require a

src/test/compile-fail/extfmt-too-many-args.rs

@@ -1,5 +1,7 @@
 // error-pattern:too many arguments
 
+// xfail-test (issue #936)
+
 use std;
 
 fn main() { let s = #fmt["%s", "test", "test"]; }