Test: Add platform helpers

Add helpers related to the current platform to assist with conditionally
running test cases.

Sources/_InternalTestSupport/PlatformHelpers.swift

@@ -0,0 +1,53 @@
+
+
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+import Basics
+
+import Testing
+
+public func isWindows() -> Bool {
+    #if os(Windows)
+    return true
+    #else
+    return false
+    #endif
+}
+
+public func isLinux() -> Bool {
+    #if os(Linux)
+    return true
+    #else
+    return false
+    #endif
+}
+
+public func isMacOS() -> Bool {
+    #if os(macOS)
+    return true
+    #else
+    return false
+    #endif
+}
+
+public func isRealSigningIdentityTestEnabled() -> Bool {
+    #if ENABLE_REAL_SIGNING_IDENTITY_TEST
+    return true
+    #else
+    return false
+    #endif
+}
+
+public func isEnvironmentVariableSet(_ variableName: EnvironmentKey) -> Bool {
+    guard let value = Environment.current[variableName] else { return false }
+    return !value.isEmpty
+}

Tests/_InternalTestSupportTests/PlatformHelpersTests.swift

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import _InternalTestSupport
+import Basics
+import Testing
+
+struct testisEnvironmentVariableSet {
+    @Test(
+        arguments: [
+            (name: "", expected: false),
+            (name: "DOES_NOT_EXIST", expected: false),
+            (name: "HOME", expected: true)
+        ]
+    )
+    func testisEnvironmentVariableSetReturnsExpectedValue(name: String, expected: Bool) {
+        // GIVEN we have an environment variable name
+        let variableName = EnvironmentKey(name)
+
+        // WHEN we call isEnvironmentVariableSet(varaiblename)
+        let actual = isEnvironmentVariableSet(variableName)
+
+        // THEN we expect to return true
+        #expect(actual == expected, "Actual is not as expected")
+    }
+}