test: cover compatibility function return types

tests/typecheck_function_compat_return_bad.claro

@@ -0,0 +1,6 @@
+TEACH square TAKES amount RETURNS NUMBER
+    CHECK TYPE amount IS NUMBER
+    RETURN "oops"
+LEARNED
+
+CALL square WITH 4

tests/typecheck_function_compat_return_good.claro

@@ -0,0 +1,6 @@
+TEACH square TAKES amount RETURNS NUMBER
+    CHECK TYPE amount IS NUMBER
+    RETURN amount
+LEARNED
+
+CALL square WITH 4