rustfmt: Drop util/test_channel_signer.rs from exclusion list

Author: tnull

rustfmt_excluded_files

@@ -75,7 +75,6 @@ lightning/src/util/invoice.rs
 lightning/src/util/message_signing.rs
 lightning/src/util/mod.rs
 lightning/src/util/string.rs
-lightning/src/util/test_channel_signer.rs
 lightning/src/util/test_utils.rs
 lightning/src/util/time.rs
 lightning/src/util/transaction_utils.rs