Disable collecting test coverage by default

[bouweandela]

Description

Collecting code coverage is time consuming. Disable it by default and enable it on request by running pytest --cov. This will save some time when developing locally and also reduces the runtime of the GitHub Actions tests by about 1 (Linux) to 2 (OSX) minutes.

Link to documentation: https://esmvaltool--2456.org.readthedocs.build/projects/ESMValCore/en/2456/contributing.html#test-coverage

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Before you get started

• [ ] ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🧪 The new functionality is relevant and scientifically sound
• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Changes are backward compatible
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

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To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov[bot]]

Codecov Report

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

Project coverage is 94.62%. Comparing base (4102388) to head (7ef0796).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2456 +/- ## ======================================= Coverage 94.62% 94.62% ======================================= Files 246 246 Lines 14055 14055 ======================================= Hits 13299 13299 Misses 756 756 ```

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