feat: Improve coverage for VNC

[fghaas]

• Add a wait_for_vnc() method (analogous to wait_for_rdp()).
• Adapt the Guacamole client so that it uses 5900 as the default VNC port.
• Add VNC coverage to tests.

[fghaas]

@mrtmm This is extremely tentative, but does the approach here look reasonable to you?

(I'm debating if this is properly feat or fix — our README does mention VNC so there's some merit in the argument that this is really a fix.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.52%. Comparing base (8ed9d35) to head (b870873).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   95.50%   95.52%   +0.02%     
==========================================
  Files          24       24              
  Lines        1979     1990      +11     
==========================================
+ Hits         1890     1901      +11     
  Misses         89       89              

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[mrtmm]

Changes look good to me; I would keep the feat, a fix in my opinion would be updating the README to document the actual feature support.