tests: basic cli/server integration tests

[orbitalturtle]

This PR puts a basic test of the server in place, testing that the cli + server can properly make a payment.

To do so required a little refactoring:

• IIUC initially a RefCell was added to the LndNodeSigner because the original version of tonic_lnd returns a mutable reference to the signer. However I don't think this reference needs to be mutable. For this PR, this was needed so we could spin up the onion messenger and server in a separate thread in the integration tests. But it's a change that could help with future efficiency as well now the onion messenger can be used in a multi-threaded way.
• Split off the server formation logic from main.rs into a separate function so that we can call it in the integration tests as well.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (1cf2906) to head (63b0bd7).

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #171   +/-   ##
======================================
  Coverage    0.00%   0.00%           
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  Files           1       1           
  Lines         126      92   -34     
======================================
+ Misses        126      92   -34     

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[dunxen]

Finally lol