Test cases

- Added expression test for `mixins/file.t` by using another embedding of the file `mixins/file.t`

source/tlang/testing/mixins/decl2.txt

@@ -0,0 +1 @@
+myVarName

source/tlang/testing/mixins/file.t

@@ -3,4 +3,7 @@ module file;
 void main()
 {
 	embed("source/tlang/testing/mixins/decl.txt");
+
+	int myVarName;
+	myVarName = embed("source/tlang/testing/mixins/decl2.txt")+1;
 }