Adds option to specify a function for a client password. When the client
is connected, if the value of password is a function then it is invoked
to get the password to use for that connection.

The function must return either a string or a Promise that resolves to
a string. If the function throws or rejects with an error then it will
be bubbled up to the client.

Add testAsync() helper function to Suite to simplify running tests that
return a Promise. The test action is executed and if a syncronous error
is thrown then it is immediately considered failed. If the Promise resolves
successfully then the test is considered successful. If the Promise
rejects with an Error then the test is considered failed.