Isolate tests that uses external_fixturepkg into a venv

[lieryan]

Description

Isolate tests that uses external_fixturepkg into a venv

This avoids modifying the test runtime environment itself.

Fixes #781

Checklist (delete if not relevant):

• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have updated CHANGELOG.md

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.86%. Comparing base (1489d32) to head (aa0ffa6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #783 +/- ## ======================================= Coverage 94.86% 94.86% ======================================= Files 135 135 Lines 26383 26405 +22 ======================================= + Hits 25028 25050 +22 Misses 1355 1355 ```

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