[validation-test] avoid crash in StringUTF8

[omochi]

To avoid crash, it uses optional binding.

Resolves half: https://bugs.swift.org/browse/SR-12577

Test case fail as usual, but it will be improved so that detailed output can be obtained.

Example:

[ RUN      ] StringUTF8Tests.Contiguous Access
stdout>>> abcd
stdout>>> abcdefghijklmnop
stdout>>> check failed at /Users/omochi/work/swift-source/swift/validation-test/stdlib/StringUTF8.swift, line 69
stdout>>> expected: true
stdout>>> check failed at /Users/omochi/work/swift-source/swift/validation-test/stdlib/StringUTF8.swift, line 72
stdout>>> expected optional to be non-nil
stdout>>> abcdéfghijk
stdout>>> á
stdout>>> 👻
stdout>>> Spooky long string. 👻
stdout>>> в чащах юга жил-был цитрус? да, но фальшивый экземпляр
stdout>>> 日

@milseman @Catfish-Man Could you review this?

[omochi]

@swift-ci Please smoke test

[milseman]

@Catfish-Man, could you review?

[milseman]

@swift-ci please test

[milseman]

Full test is needed for validation tests

[omochi]

OK. I will do so in next time.

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)