make MockHashAlgorithm thread safe

Author: tomerd

Sources/SPMTestSupport/MockHashAlgorithm.swift

@@ -8,12 +8,13 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
+import Basics
 import TSCBasic
 
 public class MockHashAlgorithm: HashAlgorithm {
     public typealias Hash = (ByteString) -> ByteString
 
-    public private(set) var hashes: [ByteString] = []
+    public private(set) var hashes = ThreadSafeArrayStore<ByteString>()
     private var hashFunction: Hash!
 
     public init(hash: Hash? = nil) {