feat: Add ClearInitializers and ClearFinalizers method

[Kirdock]

These methods let you clear any set initializer and finalizer.

Why this change?

You may want to start tests cleanly. TestA could add finalizers, and another one, TestB, could also add some, but then TestB also has and executes TestA's finalizers.

[CLAassistant]

All committers have signed the CLA.

[ccoVeille]

I'm unsure how initializers and finalizers are used.

I'm surprised they are global, and the fact we could face race issues.

I would have expected these methods signatures to use *testing.T as argument, so a t.Cleanup could have been used with OnFinalize and OnFinalize, but I'm unsure if these methods are only related to existing tests.

So here, I'm unsure I would have coded the initializer and finalizers like this.

But these considerations are with pre-existing code, the signatures are public, they cannot be changed.

Let's go back to your PR now, it provides a solution for a problem, so I would say why not.

@marckhouzam might provide a better feedback than mine, but for now, and based on my understanding I approve this PR

[Kirdock]

@ccoVeille had the same thought, so does anything speak against having a Finalize function on a command that is deferred right after this line or after retrieving the flags?

[Kirdock]

Follow-up: #2276

[ccoVeille]

@ccoVeille had the same thought, so does anything speak against having a Finalize function on a command that is deferred right after this line or after retrieving the flags?

Nothing I know, maybe @marckhouzam will have some thoughts about it