Move tests in `channelmanager.rs` into more appropriate test files

[dunxen]

The channelmanager.rs file is huuuge³ at the moment and we shouldn't be adding more tests to it. We should move them into more appropriate test files (which may involve creating new _test.rs files too.

This is unlikely to create a ton of git noise as it should just be pure moves of tests so it could probably be done in one PR.