Combine similar tests for const match

See https://github.com/rust-lang/rust/pull/66788#issuecomment-558799307
for context.

src/test/ui/consts/control-flow/exhaustive-c-like-enum-match.rs

@@ -18,4 +18,14 @@ const fn f(e: E) {
     }
 }
 
-fn main() {}
+const fn g(e: E) -> usize {
+    match e {
+        _ => 0
+    }
+}
+
+fn main() {
+    const X: usize = g(E::C);
+    assert_eq!(X, 0);
+    assert_eq!(g(E::A), 0);
+}

src/test/ui/consts/control-flow/single-arm-match-wild.rs

@@ -1,21 +0,0 @@
-// check-pass
-
-#![feature(const_if_match)]
-
-enum E {
-    A,
-    B,
-    C
-}
-
-const fn f(e: E) -> usize {
-    match e {
-        _ => 0
-    }
-}
-
-fn main() {
-    const X: usize = f(E::C);
-    assert_eq!(X, 0);
-    assert_eq!(f(E::A), 0);
-}