[Tests] NFC: Add a package loading test for .defaultIsolation setting

Meant to be a part of #8301

Author: xedin

Tests/PackageLoadingTests/PD_6_2_LoadingTests.swift

@@ -0,0 +1,56 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2023-2025 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 PackageModel
+import SourceControl
+import XCTest
+
+final class PackageDescription6_2LoadingTests: PackageDescriptionLoadingTests {
+    override var toolsVersion: ToolsVersion {
+        .v6_2
+    }
+
+    func testDefaultIsolationPerTarget() async throws {
+        let content = """
+        import PackageDescription
+        let package = Package(
+            name: "Foo",
+            defaultLocalization: "be",
+            products: [],
+            targets: [
+                .target(
+                    name: "Foo",
+                    swiftSettings: [
+                        .defaultIsolation(nil)
+                    ]
+                ),
+                .target(
+                    name: "Bar",
+                    swiftSettings: [
+                        .defaultIsolation(MainActor.self)
+                    ]
+                )
+            ]
+        )
+        """
+
+        let observability = ObservabilitySystem.makeForTesting()
+        let (_, validationDiagnostics) = try await loadAndValidateManifest(
+            content,
+            observabilityScope: observability.topScope
+        )
+        XCTAssertNoDiagnostics(validationDiagnostics)
+        XCTAssertNoDiagnostics(observability.diagnostics)
+    }
+}