Add support for retrieving and suspending the channel signer

Add support for retrieving and suspending the channel signer

This adds a `get_signer` method to the context so that a test can get ahold of
the channel signer. Adds a `set_available` method on the `TestChannelSigner`
to allow a test to enable and disable the signer: when disabled some of the
signer's methods will return `Err` which will typically activate the error
handling case. Adds a `set_channel_signer_available` function on the test
`Node` class to make it easy to enable and disable a specific signer.

Fix fuzz test