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[bot]]

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 :warning:
cmd/exit_broadcast.go	9.09%	9 Missing and 1 partial :warning:
cmd/exit_fetch.go	10.00%	8 Missing and 1 partial :warning:
cmd/exit_list.go	10.00%	8 Missing and 1 partial :warning:
cmd/exit_sign.go	10.00%	8 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ 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 ```

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[sonarcloud[bot]]

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