Silence irrelevant NPM errors when a test fails. See e.g. https://github.com/FreshRSS/FreshRSS/runs/3999501244?check_suite_focus=true
```text
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! freshrss@ markdownlint: `markdownlint '**/*.md'`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the freshrss@ markdownlint script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
npm ERR! A complete log of this run can be found in:
npm ERR! /home/runner/.npm/_logs/2021-10-25T16_15_34_166Z-debug.log
```
Efforts to reduce the resources used by CI:
* Only one git checkout
* Exclusion of irrelevant directories for several commands
* Fix some rtlcss warnings
* Move some commands from tests.yml to composer.json to make them reusasble
* Initial efforts to take avantage of all that from `make` (help welcome)
* Improved markdownlint
* Relaxed rules slighlty
* `npm run markdownlint` for automatic tests
* `npm run markdownlint_fix` for automatic syntax fixing
* Applied the fixes on all our Markdown files
