Fix backwards logic on --use-legacy-import and add tests

[dbarnett]

Fixes #836 (or at least improves it).

---

This pull request enhances the testing infrastructure for gcalcli to more accurately simulate Google Calendar API interactions, particularly for the ImportICS functionality.

Key changes include:

• Improved Google API Client Mocking: The test utilities (tests/_utils.py and tests/conftest.py) have been updated to faithfully mimic the Google API client's two-step call pattern. This involves methods (like list(), insert(), import_()) returning a request object, which then requires an execute() call to perform the actual API operation. This ensures that tests precisely track which Google API method is invoked.
• Enhanced ImportICS Test Accuracy: This improved mocking allows for more accurate verification of gcalcli's ImportICS method, ensuring it correctly dispatches to either the Google Calendar events().import_ or events().insert API based on event properties and the --use-legacy-import flag.
• Test Variable Renaming: In tests/test_gcalcli.py, the ics_content variable was renamed to ics_file in relevant ImportICS tests for improved clarity.

These changes provide a more robust testing environment to verify the correct logic for using the import versus insert API, especially in the context of the --use-legacy-import flag.

[dbarnett]

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