Add UI test for cfg!(foo, bar)

src/test/ui/macros/cfg.rs

@@ -2,4 +2,5 @@ fn main() {
     cfg!(); //~ ERROR macro requires a cfg-pattern
     cfg!(123); //~ ERROR expected identifier
     cfg!(foo = 123); //~ ERROR literal in `cfg` predicate value must be a string
+    cfg!(foo, bar); //~ ERROR expected 1 cfg-pattern
 }

src/test/ui/macros/cfg.stderr

@@ -18,6 +18,14 @@ error[E0565]: literal in `cfg` predicate value must be a string
 LL |     cfg!(foo = 123);
    |                ^^^
 
-error: aborting due to 3 previous errors
+error: expected 1 cfg-pattern
+  --> $DIR/cfg.rs:5:5
+   |
+LL |     cfg!(foo, bar);
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `cfg` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0565`.