Swap `UserConfig::default()` for `test_default_channel_config`

Swap `UserConfig::default()` for `test_default_channel_config`

As LDK changes, `UserConfig::default()` may imply marginally
different behavior, whereas `test_default_channel_config` is
intended to tweak defaults to provide a stable behavior for test
contexts.

This commit changes a few uses of `UserConfig::default()` to
`test_default_channel_config` in cases that will fail over the
coming commits due to dust changes.