Fix 6c -> carbon ext (#536)

Chandler reminded me of this, the `.carbon` extension is noted here:
https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design/code_and_name_organization#overview

executable_semantics/testdata/global_variable5.carbon

@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Test overshadowing of global variable.
+
+var Int x = 1;
+
+fn identity(Int x) {
+  return x;
+}
+
+fn main() -> Int {
+  return identity(0);
+}