Introducing an `autouse` fixture to (re)set config options for all tests.

[hendrikmakait]

In #6075, we deactivated profiling for a number of tests using the _reconfigure() context manager. In the same function, we also set other config options. I think that we would benefit from having a means of (re)setting config options globally.

For this purpose, I propose an autouse fixture that (re)sets config options for all tests, which could be _reconfigure().

I am open to alternative ways of setting config options for all tests.

[hendrikmakait]

As @gjoseph92 discovered in #6731 (comment), _reconfigure in its current state introduces flakiness in our CI pipeline. After an offline discussion with @graingert, we decided that instead of implicitly disabling profiling for tests which import our autouse fixture, it would be preferable to issue a warning whenever profiling is running in a test (#6819). In light of #6806, this would have the benefit that it also applies to tests that do not import the fixture and alert those users that they likely want to deactivate profiling for their tests.

lf the need arises for other config options to be set for all tests, we can revisit introducing some autouse fixture but for now I am closing this as the main purpose would have been to deactivate profiling.