Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

This PR changes the way integration tests are run in the CI so it takes less time to run the whole tests suite.
This greatly enhances the contributors' experience so they don't have to wait almost an hour to merge a PR, or even more in case of failures.

• What is the current behavior?

Integration tests are run concurrently two at a time per runner.

• What is the new behavior?

Each runner now runs just a single file, this cuts the execution time of the full tests suite more than half.

test_compile.py has been split into 4 parts since it was taking quite a bit to run even with this solution.

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

None.

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