More datagram tests

[swhitty]

Updates AsyncSocket.Message to expose its bytes as payload: Data.

Extends the existing datagram tests to better cover sendMessage().

For these datagram tests:

• Darwin uses AF_INET6 sockets
• Linux uses AF_UNIX pair.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.77%. Comparing base (13778ca) to head (02e89e5).

Files with missing lines	Patch %	Lines
FlyingSocks/Sources/AsyncSocket.swift	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   92.43%   94.77%   +2.34%     
==========================================
  Files          60       60              
  Lines        3938     3943       +5     
==========================================
+ Hits         3640     3737      +97     
+ Misses        298      206      -92     

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