Remove 'try XCTSkip()' from functional tests

[mattt]

This PR fixes the following warnings when building the project with Xcode 12:

No calls to throwing functions occur within 'try' expression

Fix warning: "Result of 'XCTSkip' initializer is unused"

I think the intention was for these to be throw XCTSkip() to skip these tests (#2948), but as far as I can tell, this change wouldn't have that effect. If the parallel tests are no longer causing CI failures, I think we should be able to remove these outright.

[MaxDesiatov]

@swift-ci please smoke test

[abertelrud]

IIRC these tests were skipped because they were causing occasional failures in full Swift CI. Boris may know more about the details, but I don't know that we can no longer skip them. But if they were never really skipped in the first place, that may put things in a different light.

[abertelrud]

So XCTSkip() conforms to Error, so I think the intent is to do throw XCTSkip(). But that results in warnings about how the code below it doesn't get executed (which is not wrong, but is intentional in this case). The ergonomics here seem slightly lacking.

[neonichu]

Thanks for noticing this, I mean to use try XCTSkipIf(true) to avoid the error, but instead did this :/ We do still see issues with this and need to actually skip a few more tests which also run an inferior swift test. I'll fix this and disable the additional tests in a separate PR.

[neonichu]

Closing on favour of #2984