Implement DerefMut for Once fixtures, to allow mutable access to non-Clone fixture resources

[dantliff-sqc]

Typically, if a test needs mutable access to a fixture, the test can clone the fixture value and rebind it as mutable:

#[test]
fn test_something(my_fixture: MyFixture) {
    let mut my_fixture = (*my_fixture).clone();
    my_fixture.set_something(42);
    // ...

This is necessary when rustest is used in an embedded application, where access to hardware may require mutable access to a resource held by the fixture (e.g. an I2C bus).

Unfortunately, mostly due to HAL restrictions, sometimes a fixture isn't trivially Clone, so this is not possible without additional interior mutability wrappers around the resources. This is significant additional complexity just to provide test ability.

This PR introduces a simple implementation of DerefMut for Once fixtures, avoiding a .clone() and allowing this:

#[test]
fn test_something(mut my_fixture: MyFixture) {
    my_fixture.set_something(42);
    // ...

It might make some sense for shared or global fixtures to be DerefMut too, but the solution is more complex, and will involve locking. So only Once fixtures have this trait implemented by the #[fixture] macro in this PR.

[mgautierfr]

This is nice. Thanks for the PR.

Maybe few improvement of the tests but else I agree with the feature.
Can you also update the documentation (in rustest/src/lib.rs and Readme) ?

[dantliff-sqc]

Will update docs also

[dantliff-sqc]

Sorry, not sure what status report is awaited...

[mgautierfr]

Sorry, not sure what status report is awaited...

Sorry, I have somehow missed that you update the PR and forget about it.
I will be more attentive next time.

[mgautierfr]

not sure what status report is awaited...

For some reason, github has not triggered a workflow run on your second commit and so never see the status reported.

The change of the second commit is small enough to merge without the CI passing.

Thanks for the PR.