fix/tests

[edouard-lopez]

• test: set a value for pure_shorten_prompt_current_directory_length
• test: check _pure_prompt_first_line show kubernetes context and namespace
• test: set pure_shorten_prompt_current_directory_length to valid value
• chore: add stage to Dockerfile
• fix: improve uninstallation script
• test: set pure_shorten_window_title_current_directory_length default
• chore(test): rename hooks setup->before_all, teardown->after_all
• test: create a _spy function to log calls in /tmp/called
• docs: update _mock description
• test: clean up spy call after each test
• refactor(spy): move _has_called and _cleanup_call to mocks/mocks.fish