Enforce no invalid cfg flags

...and explicitly allow `test_mod_pointers` so new rustc doesn't
spit out a ton of warnings

Author: TheBlueMatt

lightning-c-bindings/Cargo.toml

@@ -37,3 +37,9 @@ panic = "abort"
 
 [profile.release]
 panic = "abort"
+
+[lints.rust.unexpected_cfgs]
+level = "forbid"
+check-cfg = [
+    "cfg(test_mod_pointers)",
+]