Add arbitrary_self_types feature gate error to some tests

Author: mikeyhew

src/test/compile-fail/issue-26194.rs

@@ -13,6 +13,7 @@ struct S(String);
 impl S {
     fn f(self: *mut S) -> String { self.0 }
     //~^ ERROR invalid `self` type
+    //~| ERROR arbitrary `self` types are unstable
 }
 
 fn main() { S("".to_owned()).f(); }

src/test/compile-fail/ufcs-explicit-self-bad.rs

@@ -15,7 +15,9 @@ struct Foo {
 }
 
 impl Foo {
-    fn foo(self: isize, x: isize) -> isize {  //~ ERROR invalid `self` type
+    fn foo(self: isize, x: isize) -> isize {
+        //~^ ERROR invalid `self` type
+        //~| ERROR arbitrary `self` types are unstable
         self.f + x
     }
 }
@@ -25,10 +27,14 @@ struct Bar<T> {
 }
 
 impl<T> Bar<T> {
-    fn foo(self: Bar<isize>, x: isize) -> isize { //~ ERROR invalid `self` type
+    fn foo(self: Bar<isize>, x: isize) -> isize {
+        //~^ ERROR invalid `self` type
+        //~| ERROR arbitrary `self` types are unstable
         x
     }
-    fn bar(self: &Bar<usize>, x: isize) -> isize {   //~ ERROR invalid `self` type
+    fn bar(self: &Bar<usize>, x: isize) -> isize {
+        //~^ ERROR invalid `self` type
+        //~| ERROR arbitrary `self` types are unstable
         x
     }
 }