lang: Add test for unused include statements

Even if they have no side effect, they aren't legal, since it would be
surprising if suddenly something new got added in one which then broke
the imports.

lang/interpret_test/TestAstFunc1/unused2.txtar

@@ -0,0 +1,19 @@
+-- main.mcl --
+import "fmt"
+import "something.mcl"
+
+print "p1" {
+	msg => fmt.printf("someint: %d", $something.someint),
+}
+-- something.mcl --
+$someint = 42
+class someclass {
+	print "p2" {
+		msg => "i'm inside of someclass",
+	}
+}
+
+# this should generate a compile error
+include someclass
+-- OUTPUT --
+# err: errSetScope: local import of `something.mcl` failed: module contains unused statements: found stmt: include(someclass)