Add kernel tests

Added a kernel test for the basic functionality. Does not cover translations of nodes, yet. Can you please list some edge cases that were relevant in former issues?

@simonbaese Thanks again for your contributions.

The main case that caused issues was on creation of new nodes where the core links field was attempted to be rendered, but neither a node ID nor revision ID was available yet and the node couldn't be loaded. The issues involving translations was due to the same error.

Potential tests:

1. Creation of new node
2. Update the node
3. Create a new translation and test that both the translated estimated read time and source estimated read time are correct
4. Update a translation

The test is failing with the discussed cases, because there is another bug. Please see 🐛 Creating multi-language entities programmatically does not estimate read time for all translations Active . After resolving the issue, the tests will go green.

Thanks @simonbaese and @kevinquillen! With the fix merged from https://www.drupal.org/project/estimated_read_time/issues/3395919 🐛 Creating multi-language entities programmatically does not estimate read time for all translations Active , the tests passed.

Automatically closed - issue fixed for 2 weeks with no activity.