Update junit and mockito

- JUnit has been bumped to 4.13.2.
- Mockito has gone from 1.10.19 to 3.12.4, and now compiles correctly
  on OpenJDK 16 using Maven 3.8.

pom.xml

@@ -24,13 +24,13 @@
         <dependency>
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>
-            <version>4.13.1</version>
+            <version>4.13.2</version>
             <scope>test</scope>
         </dependency>
         <dependency>
             <groupId>org.mockito</groupId>
-            <artifactId>mockito-all</artifactId>
+            <artifactId>mockito-core</artifactId>
-            <version>1.10.19</version>
+            <version>3.12.4</version>
             <scope>test</scope>
         </dependency>
         <dependency>