Support custom countSpec in SimpleJpaRepository.findAll(…).

Closes #3727

Author: JoshuaChen

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/JpaSpecificationExecutor.java

@@ -37,6 +37,7 @@
 import org.springframework.data.jpa.domain.Specification;
 import org.springframework.data.jpa.domain.UpdateSpecification;
 import org.springframework.data.repository.query.FluentQuery;
+import org.springframework.lang.Nullable;
 
 /**
  * Interface to allow execution of {@link Specification}s based on the JPA criteria API.

spring-data-jpa/src/test/java/org/springframework/data/jpa/repository/UserRepositoryTests.java

@@ -47,7 +47,6 @@
 import org.assertj.core.api.SoftAssertions;
 import org.hibernate.LazyInitializationException;
 import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;