Fix tests that rely on Exception.handled

[b2gills]

Now that you can set Exception.handled let's actually test it.

I changed the test to assign True rather than 1 so that in the future it could be implemented with has bool $.handled is rw; rather than as a proxied rw method which it is currently.

While looking into this I found two tests that were looking at $!.pending even though $! should be empty at that point. I think it was just a copy paste error. I changed one to look at @fails instead, and removed the other one.

Since all of the tests I changed were skipped or broken anyway, there should be no problems with maintaining compatibility with v6.c.

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Actually thinking about it .handled might be better if it was called .is-handled as someone could think that just calling .handled causes it to be handled.

It should be safe to do this as the method was never there to begin with.

[salortiz]

"S04/Exceptions" talks about @! as an unthrown exceptions list of Failures, so my though was that this S04-exceptions/pending.t tests was, in principle, about them.

I assumed that if $! !~~ Exception, then it should be Failure, and .pending the planned way to access that list. (Currently $! is Exception only after a failed try block)

That way, all tests made sense.

About .handled, in S04 it is described as a method of Mu only overridden by Failure and I start working on it when I found it described in standard documentation, see https://github.com/perl6/doc/blob/master/doc/Type/Failure.pod#L31, try to use it and failed.

Its evident that S04 and the current implementation are diverging, so I take roast as normative, and this PR removes that way for testing pending Failures. I that isn't desired I haven't drawbacks.

(Mu.handled is trivial, but I don't see its case.)

[JJ]

Looks like this is still needed, but can someone please check the conflicts?