test: Use serial dispatch queue for invocations (#3981)

Using the barrier flag with the concurrent attribute is similar to using
a serial dispatch queue, so we can use a serial dispatch queue instead
to simplify the syntax.

SentryTestUtils/Invocations.swift

@@ -7,7 +7,7 @@ public class Invocations<T> {
 
     public init() {}
 
-    private let queue = DispatchQueue(label: "Invocations", attributes: .concurrent)
+    private let queue = DispatchQueue(label: "Invocations")
 
     private var _invocations: [T] = []
 
@@ -42,13 +42,13 @@ public class Invocations<T> {
     }
 
     public func record(_ invocation: T) {
-        queue.async(flags: .barrier) {
+        queue.async {
             self._invocations.append(invocation)
         }
     }
 
     public func removeAll() {
-        queue.async(flags: .barrier) {
+        queue.async {
             self._invocations.removeAll()
         }
     }