Update availability annotations in tests

[simonjbeaumont]

Checklist

• [x] I've run tests to see all new and existing tests pass
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

~If you've made changes to gyb files~

• [ ] ~I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request~

Motivation:

When building the tests for iOS and tvOS, the build fails because we are using APIs that aren't available on all platform versions. E.g.:

Tests/CryptoTests/Digests/DigestsTests.swift:117:37: error: 'blockByteCount' is only available in iOS 13.2 or newer
        XCTAssertEqual(Insecure.MD5.blockByteCount, 64)

Modifications:

• Add @available guard to test that uses blockByteCount API.
• Update DataProtocol.hexString to not use String.init(unsafeUninitializedCapacity:initializingUTF8With:), which is only available on iOS 14+, and instead use Array.init(unsafeUninitializedCapacity:initializingWith:), which is available on older platforms.

Result:

Tests can now build for and run on iOS and tvOS.

[Lukasa]

@swift-server-bot add to allowlist