[Heap] Enable heap tests in optimized builds (apple#101)

@testable import is replaced with @_spi(Testing) import.
A test for _Node is moved to another file because this test needs access to internals of PriorityQueueModule and this test will not work in optimized builds.

Tests/PriorityQueueTests/HeapTests.swift

@@ -9,9 +9,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if DEBUG
 import XCTest
-@testable import PriorityQueueModule
+@_spi(Testing) import PriorityQueueModule
 
 final class HeapTests: XCTestCase {
   func test_isEmpty() {
@@ -349,19 +348,6 @@ final class HeapTests: XCTestCase {
 
   // MARK: -
 
-  func test_levelCalculation() {
-    // Check alternating min and max levels in the heap
-    var isMin = true
-    for exp in 0...12 {
-      // Check [2^exp, 2^(exp + 1))
-      for offset in Int(pow(2, Double(exp)) - 1)..<Int(pow(2, Double(exp + 1)) - 1) {
-        let node = _Node(offset: offset)
-        XCTAssertEqual(node.isMinLevel, isMin)
-      }
-      isMin.toggle()
-    }
-  }
-
   func test_initializer_fromCollection() {
     var heap = Heap((1...20).shuffled())
     XCTAssertEqual(heap.max(), 20)
@@ -420,4 +406,3 @@ final class HeapTests: XCTestCase {
     }
   }
 }
-#endif

Tests/PriorityQueueTests/NodeTests.swift

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Collections open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+#if DEBUG // These unit tests need access to PriorityQueueModule internals
+import XCTest
+@_spi(Testing) @testable import PriorityQueueModule
+
+class NodeTests: XCTestCase {
+  func test_levelCalculation() {
+    // Check alternating min and max levels in the heap
+    var isMin = true
+    for exp in 0...12 {
+      // Check [2^exp, 2^(exp + 1))
+      for offset in Int(pow(2, Double(exp)) - 1)..<Int(pow(2, Double(exp + 1)) - 1) {
+        let node = _Node(offset: offset)
+        XCTAssertEqual(node.isMinLevel, isMin)
+      }
+      isMin.toggle()
+    }
+  }
+}
+#endif // DEBUG