Stop mocking class that is final in latest Framework 5.1 snapshots

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/client/RootUriRequestExpectationManagerTests.java

@@ -86,7 +86,7 @@ public void createWhenExpectationManagerIsNullShouldThrowException() {
 
 	@Test
 	public void expectRequestShouldDelegateToExpectationManager() {
-		ExpectedCount count = mock(ExpectedCount.class);
+		ExpectedCount count = ExpectedCount.once();
 		RequestMatcher requestMatcher = mock(RequestMatcher.class);
 		this.manager.expectRequest(count, requestMatcher);
 		verify(this.delegate).expectRequest(count, requestMatcher);