CP-46136: Add doc/testing.md: Describe the used test approach

[bernhardkaindl]

For you @ashwin9390!

Improve README-pytest.md:

• Explain integration testing better
• Fix some spelling and grammar errors

[coveralls]

Pull Request Test Coverage Report for Build 7980415564

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.471%

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Totals
Change from base Build 7931834360:	0.0%
Covered Lines:	1946
Relevant Lines:	2175

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💛 - Coveralls

[codecov[bot]]

Codecov Report

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

Comparison is base (5ade7b7) 86.96% compared to head (c153505) 86.95%. Report is 10 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #83 +/- ## ========================================== - Coverage 86.96% 86.95% -0.02% ========================================== Files 17 17 Lines 2141 2138 -3 ========================================== - Hits 1862 1859 -3 Misses 279 279 ``` | [Flag](https://app.codecov.io/gh/xenserver/status-report/pull/83/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xenserver) | Coverage Δ | | |---|---|---| | [python2.7](https://app.codecov.io/gh/xenserver/status-report/pull/83/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xenserver) | `85.62% <ø> (+0.02%)` | :arrow_up: | | [python3.10.13](https://app.codecov.io/gh/xenserver/status-report/pull/83/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xenserver) | `86.30% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=xenserver#carryforward-flags-in-the-pull-request-comment) to find out more.

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