add trapping double test

Tests/SwiftFusionTests/Core/TrappingDoubleTests.swift

@@ -0,0 +1,37 @@
+// Copyright 2020 The SwiftFusion Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import XCTest
+
+import PenguinStructures
+import SwiftFusion
+
+class TrappingDoubleTests: XCTestCase {
+  /// Tests simple `TrappingDouble` operations.
+  func testOperations() {
+    let x: Double = 1.0
+    let y: Double = 2.0
+    XCTAssertEqual(y + x, 3.0)
+    XCTAssertEqual(y - x, 1.0)
+  }
+
+  /// Tests that `TrappingDouble` traps on `NaN`s.
+  ///
+  /// There is no way to assert that a trap happens, so this test is skipped. You can remove the
+  /// `XCTSkipIf` line and run this test to manually verify the trapping behavior.
+  func testTrap() throws {
+    try XCTSkipIf(true)
+    let _: Double = .infinity - .infinity
+  }
+}