Drop a test.

[thomasvl]

This test was leveraging a questionable behavior of Foundation. The input isn't valid UTF-8, and there was a silent substitution going on, but trunk versions of the compiler (and hence Foundation) properly fail to make a String out of the input, so the force unwrap fails. So just drop the test as no longer valid.

[allevato]

For posterity, this change in swift-corelibs-foundation started causing this to fail (it wasn't part of the 5.10 release, I caught it from our internal builds that track main-ish). Instead of the invalid UTF-8 code units being substituted (with U+FFFD) as they were previously (and still are on Darwin, at least in macOS 14.4), the initializer returned nil instead.

I'm not sure if that's an intentional change in swift-corelibs-foundation, but the documentation for String(bytes:encoding:) says nothing about its substitution behavior, so it's best we don't rely on it.