Fix the overflowing tests in run-fail.

* The error patterns had a typo.

* Our current constant evaluation would silently allow the overflow
  (filed as Issue 22531).

* The overflowing-mul test was accidentally doing addition instead of
  multiplication.

Author: pnkfelix

src/test/run-fail/overflowing-add.rs

@@ -8,8 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:thread '<main>' panicked at 'arithmatic operation overflowed'
+// error-pattern:thread '<main>' panicked at 'arithmetic operation overflowed'
+
+// (Work around constant-evaluation)
+fn value() -> u8 { 200 }
 
 fn main() {
-    let x = 200u8 + 200u8 + 200u8;
+    let _x = value() + value() + value();
 }

src/test/run-fail/overflowing-mul.rs

@@ -8,8 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:thread '<main>' panicked at 'arithmatic operation overflowed'
+// error-pattern:thread '<main>' panicked at 'arithmetic operation overflowed'
+
+// (Work around constant-evaluation)
+fn value() -> u8 { 200 }
 
 fn main() {
-    let x = 200u8 + 4u8;
+    let x = value() * 4;
 }

src/test/run-fail/overflowing-sub.rs

@@ -8,8 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:thread '<main>' panicked at 'arithmatic operation overflowed'
+// error-pattern:thread '<main>' panicked at 'arithmetic operation overflowed'
+
+// (Work around constant-evaluation)
+fn value() -> u8 { 42 }
 
 fn main() {
-    let x = 42u8 - 43u8;
+    let _x = value() - (value() + 1);
 }