Tests: compute the TestInputs path more robustly

This adjusts the computation of the `TestInputs` path by walking the
components in reverse.  Doing so ensures that we can use the native path
separator character, which is required for the proper behaviour on
Windows.  With this change we now pass additional tests in the
SwiftDriverTests test suite.

Author: compnerd

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -35,6 +35,14 @@ private func rebase(_ arcs: String..., at base: AbsolutePath) -> String {
   base.appending(components: arcs).pathString.nativePathString()
 }
 
+private var testInputsPath: AbsolutePath = {
+  var root: AbsolutePath = AbsolutePath(#file)
+  while root.basename != "Tests" {
+    root = root.parentDirectory
+  }
+  return root.parentDirectory.appending(component: "TestInputs")
+}()
+
 final class SwiftDriverTests: XCTestCase {
 
   private var envWithFakeSwiftHelp: [String: String] {
@@ -3426,12 +3434,9 @@ final class SwiftDriverTests: XCTestCase {
 
   func testDarwinSDKTooOld() throws {
     func getSDKPath(sdkDirName: String) -> AbsolutePath {
-      let packageRootPath = AbsolutePath(String(URL(fileURLWithPath: #file).pathComponents
-          .prefix(while: { $0 != "Tests" }).joined(separator: "/").dropFirst()))
-      let testInputsPath = packageRootPath.appending(component: "TestInputs")
-                                          .appending(component: "SDKChecks")
-      return testInputsPath.appending(component: sdkDirName)
+      return testInputsPath.appending(component: "SDKChecks").appending(component: sdkDirName)
     }
+
     // Ensure an error is emitted for an unsupported SDK
     func checkSDKUnsupported(sdkDirName: String)
     throws {
@@ -5375,10 +5380,8 @@ final class SwiftDriverTests: XCTestCase {
   }
 
   func testPrebuiltModuleCacheFlags() throws {
-    let packageRootPath = URL(fileURLWithPath: #file).pathComponents
-      .prefix(while: { $0 != "Tests" }).joined(separator: "/").dropFirst()
-    let testInputsPath = packageRootPath + "/TestInputs"
-    let mockSDKPath : String = testInputsPath + "/mock-sdk.sdk"
+    let mockSDKPath: String =
+        testInputsPath.appending(component: "mock-sdk.sdk").pathString.nativePathString()
 
     do {
       var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-ios13.1-macabi", "foo.swift", "-sdk", mockSDKPath])