Testing: Find swiftpm-testing-helper based on SwiftSDK root paths

The `swiftpm-testing-helper` itself is merged separately by
#7771

Author: xedin

Sources/Commands/SwiftTestCommand.swift

@@ -886,13 +886,8 @@ final class TestRunner {
             }
             args += [xctestPath.pathString]
         case .swiftTesting:
-            // FIXME: better way to get path to self
-            let toolPath = String(unsafeUninitializedCapacity: 2048) { buffer in
-                var count = UInt32(buffer.count)
-                _NSGetExecutablePath(buffer.baseAddress!, &count)
-                return Int(count)
-            }
-            args += [toolPath, "--test-bundle-path", testPath.pathString]
+            let helper = try self.toolchain.getSwiftTestingHelper()
+            args += [helper.pathString, "--test-bundle-path", testPath.pathString]
         }
         args += additionalArguments
         args += [testPath.pathString]

Sources/PackageModel/UserToolchain.swift

@@ -387,6 +387,24 @@ public final class UserToolchain: Toolchain {
         )
     }
 
+#if os(macOS)
+    public func getSwiftTestingHelper() throws -> AbsolutePath {
+        // The helper would be located in `.build/<config>` directory when
+        // SwiftPM is built locally and `usr/libexec/swift/pm` directory in
+        // an installed version.
+        let binDirectories = self.swiftSDK.toolset.rootPaths +
+            self.swiftSDK.toolset.rootPaths.map {
+                $0.parentDirectory.appending(components: ["libexec", "swift", "pm"])
+            }
+
+        return try UserToolchain.getTool(
+            "swiftpm-testing-helper",
+            binDirectories: binDirectories,
+            fileSystem: self.fileSystem
+        )
+    }
+#endif
+
     internal static func deriveSwiftCFlags(
         triple: Triple,
         swiftSDK: SwiftSDK,