Add basic async signer tests

Add basic async signer tests

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.

Adds a new `async_signer_tests` module:

* Check for asynchronous handling of `funding_created` and `funding_signed`.
* Check that we correctly resume processing after awaiting an asynchronous
  signature for a `commitment_signed` event.
* Verify correct handling during peer disconnect.
* Verify correct handling for inbound zero-conf.