Exclude tests that need plist serialization from linux

This need a fix that is in swift 6.1 to pass on linux:
swiftlang/swift-foundation#1002

Author: waltflanagan

Tests/XcodeProjTests/Objects/Project/PBXOutputSettingsTests.swift

@@ -1,3 +1,4 @@
+#if os(macOS) || (os(Linux) && compiler(>=6.1))
 
 import XCTest
 @testable import XcodeProj
@@ -274,3 +275,4 @@ class PBXOutputSettingsTests: XCTestCase {
         )
     }
 }
+#endif

Tests/XcodeProjTests/Objects/Project/PBXProjEncoderTests.swift

@@ -1,4 +1,4 @@
-
+#if os(macOS) || (os(Linux) && compiler(>=6.1))
 import Foundation
 import XCTest
 @testable import XcodeProj
@@ -616,3 +616,5 @@ private extension [String] {
         }
     }
 }
+
+#endif

Tests/XcodeProjTests/Objects/Project/PBXProjIntegrationTests.swift

@@ -1,3 +1,4 @@
+#if os(macOS) || (os(Linux) && compiler(>=6.1))
 import Foundation
 import PathKit
 import XCTest
@@ -84,3 +85,5 @@ final class PBXProjIntegrationTests: XCTestCase {
         XCTAssertEqual(proj.objects.remoteSwiftPackageReferences.count, 1)
     }
 }
+
+#endif

Tests/XcodeProjTests/Project/XcodeProjIntegrationTests.swift

@@ -1,3 +1,4 @@
+#if os(macOS) || (os(Linux) && compiler(>=6.1))
 import Foundation
 import PathKit
 import XCTest
@@ -109,3 +110,5 @@ final class XcodeProjIntegrationTests: XCTestCase {
         fixturesPath() + "SynchronizedRootGroups/SynchronizedRootGroups.xcodeproj"
     }
 }
+
+#endif