I noticed there were no tests for the error scenario in the Celery
package. This commit adds a basic test, based on the previous test and
how I see other packages test the error status on the span.

Part of open-telemetry#987

In addition to setting the status on the span, also record the exception
on the span. This adds an event to the span with more details about the
error, following the format other instrumentations also use.

The celery tests failed on Python 3.11. This is most likely due to this
issue in billiard, a celery dependency, about it not working on Python
3.11 because of the error reported in the CI:
celery/billiard#377

It's been fixed in billiard 4.1.0, but celery is locked on billiard
version lower than 4, so it cannot use this version with the fix.

This issue does not arise on the Docker tests, because they use Python
3.9.16.

I've moved the error test span event assertions to the error test that
is available in the functional tests, and removed the unit test. That
way, the build will run successfully.

This was added in a recent merge commit on this PR branch.

With the move of the tests for tasks with errors to the functional
tests, remove the unit test's error task and unused imports.