Add missing availability guards in tests

[simonjbeaumont]

Motivation

Some of the test code was missing availability guards for Apple platforms, resulting in build failures for these platforms, e.g.

error: 'init(pemDocument:)' is only available in iOS 14 or newer
             let privateKeyAfterRoundtrip = try Certificate.PrivateKey(pemDocument: privateKey.serializeAsPEM())

Modifications

Add missing availability guards. I've tried to keep them as scoped as possible. Some of the tests rely on initializers in CryptoKit that are only available since iOS 14. One suite of tests used this so extensively that I made the whole suite only available on newer platforms.

Result

Tests can now build and run on iOS and other Apple platforms.

[glbrntt]

@swift-server-bot add to allowlist