Fix a clang warning for treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated

```
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
```

Author: ydah

spec/lrama/integration_spec.rb

@@ -270,7 +270,7 @@ def generate_object(grammar_file_path, c_path, obj_path, command_args: [])
   end
 
   describe "sample files" do
-    let(:c_path)   { Dir.tmpdir + "/test.c" }
+    let(:c_path)   { Dir.tmpdir + "/test.#{file_extension}" }
     let(:obj_path) { Dir.tmpdir + "/test" }
 
     describe "calc.y" do