Have the mock log facade not always print to stdout

In #463 we introduced a mock logger that implements the `log` facade in
tests. Previously, it defaulted to always print `TRACE`-level logs to
`stdout`. This however can be very spammy (especially since it includes
TRACE-level logs of `electrum_client` now).

Here, we make printing the log lines optional by `cfg`-gating it.

Author: tnull

Cargo.toml

@@ -123,4 +123,5 @@ check-cfg = [
 	"cfg(ldk_bench)",
 	"cfg(tokio_unstable)",
 	"cfg(cln_test)",
+	"cfg(ldk_node_test_log_print)",
 ]

tests/common/logging.rs

@@ -48,6 +48,7 @@ impl LogFacadeLog for MockLogFacadeLogger {
 			record.line().unwrap(),
 			record.args()
 		);
+		#[cfg(ldk_node_test_log_print)]
 		println!("{message}");
 		self.logs.lock().unwrap().push(message);
 	}