exit: add custom testnet flags for exits

[KaloyanTanev]

Add testnet flags for exits. We have testnet for other CLI commands, but not for the exits. Required so we can easily test with Kurtosis.

Fix a test in cmd/createcluster_internal_test.go as well. Add custom test dir for it.

category: misc
ticket: none

[codecov]

Codecov Report

Attention: Patch coverage is 6.55738% with 57 lines in your changes missing coverage. Please review.

Project coverage is 58.66%. Comparing base (4bbb36e) to head (2ebe6f7).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/exit.go	0.00%	20 Missing ⚠️
cmd/exit_broadcast.go	9.09%	9 Missing and 1 partial ⚠️
cmd/exit_fetch.go	10.00%	8 Missing and 1 partial ⚠️
cmd/exit_list.go	10.00%	8 Missing and 1 partial ⚠️
cmd/exit_sign.go	10.00%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3317      +/-   ##
==========================================
- Coverage   58.77%   58.66%   -0.12%     
==========================================
  Files         210      210              
  Lines       30213    30274      +61     
==========================================
+ Hits        17758    17759       +1     
- Misses      10564    10619      +55     
- Partials     1891     1896       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud