[concurrency] Use nonisolated(unsafe) to quiet an error in runAsync in the tests.

Specifically, we are already using XCTest expectations to make sure we are waiting
long enough. So it is appropriate to put nonisolated(unsafe) since we are
synchronizing the code in a way that swift concurrency does not understand.

This came up as part of fixing rdar://164042741.

Author: gottesmm

Tests/SwiftSourceKitPluginTests/SwiftSourceKitPluginTests.swift

@@ -2172,7 +2172,7 @@ private struct ExpectationNotFulfilledError: Error {}
 
 /// Run the given async block and block the current function until `body` terminates.
 private func runAsync<T: Sendable>(_ body: @escaping @Sendable () async throws -> T) throws -> T {
-  var result: Result<T, Error>!
+  nonisolated(unsafe) var result: Result<T, Error>!
   let expectation = XCTestExpectation(description: "")
   Task {
     do {