Cancellable: remove UsingToken usages in tests

[auduchinok]

Removes Cancellable.UsingToken from the public surface, since the computation now always sets the token.

[github-actions]

✅ No release notes required

[auduchinok]

Accidental comment from the release notes editor, I don't seem to be able to delete it.

[auduchinok]

So, just removing the member from the public API didn't work, as it was used in Cancellable.run which is marked inline. Removing that member produced errors about insufficiently public member being used in an inline member. I've removed the inline. @majocha Could you help me with the performance testing of this change, please?

[majocha]

@auduchinok I'm not sure about removing inline there, would it affect stack traces and debugging?
Possibly it would be better to remove use _ = Cancellable.UsingToken(ct) from run and only set the token in toAsync instead.

Somewhat related: It would be great to integrate the functionality of GuardCancellable into the cancellable CE and thus simplify the stuff that's going on in CheckDeclarations.fs

[auduchinok]

@majocha I've reverted the changes around UsingToken and run, as you experiment with them in #18285. This PR is now only about simplifying the tests.

[majocha]

So, I did some benchmark runs using FCSSourceFiles benchmark and frankly I see no difference between inlined and not inlined Cancellable.run. Looking at the original implementation:

/// Run a cancellable computation using the given cancellation tokenlet inlinerun(ct:CancellationToken)(Cancellable oper)=if ct.IsCancellationRequested then
ValueOrCancelled.Cancelled(OperationCanceledException ct)else
oper ct

it seems the idea was to just expand the cancel check inline.

[psfinaki]

/azp run

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