fix: exo CLI Bug exo config add panics when new first config without it being the default one

[natalie-o-perret]

Description

This pull request refactors and expands the test coverage for configuration handling in the CLI, especially around edge cases such as missing or broken configuration files and the absence of a default account. It introduces new end-to-end and integration tests to document and reproduce a known panic, and it improves the test infrastructure for easier discovery and execution of test scenarios. Some package renaming and cleanup are also included.

Test coverage improvements for configuration edge cases:

• Added new e2e test scripts (.txtar files) to document and reproduce bugs when the config file is missing, when an account exists without a default, and to verify correct output with a valid config file. These tests also demonstrate the workaround using the --use-account flag.
• Added an integration test (config_panic_test.go) to simulate and verify the panic when adding an account without setting it as default, which cannot be covered by non-interactive e2e tests (prompt-ui-based zone selection).

Test infrastructure improvements:

• Enhanced the local test runner to recursively discover all .txtar test scripts, allowing easier addition of new test cases without manual registration.
• Removed the obsolete isolated config file test and the API-tagged testscript runner, streamlining the test suite and focusing on relevant scenarios.

Package and code organization:

• Renamed integration test packages from integ_test to integ for consistency and to facilitate internal test suite usage.
• Cleaned up test dependencies in go.mod, removing unused packages.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block)
• Testing

Testing