[test] fix availability

[glessard]

Use the general SwiftStdlib availability rather than just macOS.

[glessard]

@swift-ci please smoke test

[j-hui]

@susmonteiro why was the @available attribute even needed in the first place? And I'm wondering whether the test should just be changed to something like this instead (similar to the test case above it):

if #available(SwiftStdlib 5.8, *) {
    VirtualMethodsTestSuite.test("C++ virtual method with complex parameter") {
        func f(simpleClass: HasDestructor, immortalClass: Immortal2) {
            immortalClass.virtualMethod(simpleClass)
        }
    }
}

[glessard]

if #available is worse, since it makes the test disappear entirely in the wrong conditions. A better approach would be the .require(.stdlib_5_8) modifier (added in #79838), which would explicitly mark the test as skipped when compiled and run with the wrong availability.

[susmonteiro]

@susmonteiro why was the @available attribute even needed in the first place?

I think it's because the struct Immortal2 has swift attributes defined in IMMORTAL_FRT, which are only available from macOS 13.3