lang: Add a bunch of new language tests

These test both graph shape consistency and single value outputs.
Eventually we want to make the graph shape tests more precise, and also
verify specific outputs how it used to be. For now, this is okay.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/interpret_test/TestAstFunc1/graph12.txtar

@@ -0,0 +1,19 @@
+-- main.mcl --
+# variable shadowing both
+# this should be okay, because var is shadowed
+$x = "hello"
+if true {
+	$x = "world"	# shadowed
+	test "t2" {
+		stringptr => $x,
+	}
+}
+test "t1" {
+	stringptr => $x,
+}
+-- OUTPUT --
+Vertex: const
+Vertex: const
+Vertex: const
+Vertex: const
+Vertex: const