test: add comments about "slow" tag

What

Registering an Alcotest test require to specify if the test is quick or slow. An Alcotest test registered as quick or slow will have the associated tag in tezt.

The slow tag have consequences on how it will be handled by the CI. But there is no documented limit between quick and slow tests.

This MR:

• defines that tests that need 2 minutes or more on the CI should be tagged as slow,
• updates the doc about CI and tags.

Why

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR