lang: Port TestAstFunc2 to txtar format

This ports TestAstFunc2 from our home-grown content storage system to
the txtar package. Since a single file can be used to represent the
entire folder hierarchy, this makes it much easier to see and edit
tests.

lang/interpret_test/TestAstFunc2/stack-overflow.txtar

@@ -0,0 +1,6 @@
+-- main.mcl --
+# this should not compile, but previously once did! (woops)
+import "fmt"
+$x = fmt.printf("TEST is %s", if $x == "b" {"a"} else {"b"} )
+-- OUTPUT --
+# err: errSetScope: recursive reference while setting scope: not a dag