Tezt: add a test that fails

What

This MR adds a test that:

• always fails;
• has tag ci_disabled;
• has some ~uses such that the test is often selected by Manifezt.

Why

To easily detect if ci_disabled tests are not actually run in the CI. Which occurred recently because an MR introduced another call to Test.run, which caused all tests to be selected (probably coupled with nomadic-labs/tezt#29 (closed)).

How

By adding the dummy test in basic.ml.

Manually testing the MR

Check the CI (this MR has tag ci--run-all-tests so Manifezt has no impact). And:

dune exec tezt/tests/main.exe -- -f basic.ml ci_disabled

should fail, while:

dune exec tezt/tests/main.exe -- -f basic.ml /ci_disabled

should not.

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