Register test observer in main queue

Fixes a crash when calling [XCTestObservationCenter addTestObserver:] that would fail an assertion: 'Test observers can only be registered and unregistered on the main thread.'

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -371,11 +371,9 @@ func sanitizePathComponent(_ string: String) -> String {
 // We need to clean counter between tests executions in order to support test-iterations.
 private class CleanCounterBetweenTestCases: NSObject, XCTestObservation {
   private static var registered = false
-  private static var registerQueue = DispatchQueue(
-    label: "co.pointfree.SnapshotTesting.testObserver")
 
   static func registerIfNeeded() {
-    registerQueue.sync {
+    DispatchQueue.main.sync {
       if !registered {
         registered = true
         XCTestObservationCenter.shared.addTestObserver(CleanCounterBetweenTestCases())