[cleanup] Improve Test Coverage

[KCarretto]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

• Adds additional testing and bumps that coverage metric.
• Adds new HTTP_LISTEN_ADDR env var for configuring http bind.
• Adds new ENABLE_TEST_RUN_AND_EXIT env var for shutting down the server immediately after starting it.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (183e36d) 69.65% compared to head (4d3a04f) 71.32%.

Files	Patch %	Lines
tavern/internal/c2/api_report_task_output.go	76.31%	6 Missing and 3 partials ⚠️
tavern/main.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
+ Coverage   69.65%   71.32%   +1.66%     
==========================================
  Files         121      122       +1     
  Lines        9531     9547      +16     
==========================================
+ Hits         6639     6809     +170     
+ Misses       2779     2607     -172     
- Partials      113      131      +18     

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[hulto]

LGTM