Disable chan announce re-broadcast when _test_utils feat is on

In our upgrade/downgrade tests we want to be able to use the
functional test utils as-is externally as a part of CI in later
LDK versions. This implies that code that is `cfg(test)` really
needs to always be `cfg(any(test, feature = "_test_utils"))`.

Sadly, trying to open multiple channels back-to-back on a node
with `_test_utils` currently fails as we re-broadcast channel
announcement messages, something the functional test utils does not
expect.

Here we update the `cfg`-flagging of that rebroadcast logic to fix
back-to-back channel opens in `_test_utils`.

This narrowly backports one hunk of the much larger upstream
commit e41e756.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -11365,7 +11365,7 @@ where
 									// Most of our tests were written when we only broadcasted
 									// `channel_announcement`s once and then never re-broadcasted
 									// them again, so disable the re-broadcasting entirely in tests
-									#[cfg(test)]
+									#[cfg(any(test, feature = "_test_utils"))]
 									{
 										should_announce = announcement_sigs.is_some();
 									}