Add explicit import of NIOCore (#503)

Motivation:

Nightly CI builders need all the imports to be correct, and we were
missing a NIOCore import in the tests.

Modifications:

import NIOCore

Result:

Nice builds in the 6.1 nightlies

Author: Lukasa

Tests/NIOSSLTests/SecurityFrameworkVerificationTests.swift

@@ -12,6 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import NIOCore
 import XCTest
 
 @testable import NIOSSL