[ffigen] Fix a naming conflict in function internal variables

[liamappelbe]

The bindings for function foo define internal variables _foo and _fooPtr, so if there's also a fooPtr function, the names collide. This collision was not being automatically resolved because the variables were created in the function's local scope instead of the root scope.

Unrelated change: I noticed that part of the method test was commented out, so I've re-enabled it.

Fixes #2760

[github-actions]

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[coveralls]

coverage: 87.278%. remained the same
when pulling 6303be9 on ffigen_fn_conflict
into d2504b3 on main.