Upgraded mockk from 1.9.3 to 1.13.2 (#523)

Keeping up with the latest changes.

pom.xml

@@ -58,7 +58,6 @@
     <license-maven-plugin.version>1.20</license-maven-plugin.version>
     <maven.compiler.release>8</maven.compiler.release>
     <maven.version>3.5.4</maven.version>
-    <mockk.version>1.9.3</mockk.version>
     <plexus-containers.version>2.1.1</plexus-containers.version>
   </properties>
 
@@ -410,14 +409,19 @@
     </dependency>
     <dependency>
       <groupId>io.mockk</groupId>
-      <artifactId>mockk</artifactId>
+      <artifactId>mockk-jvm</artifactId>
       <scope>test</scope>
       <exclusions>
         <exclusion>
           <!-- provided by org.slf4j:log4j-over-slf4j -->
           <groupId>log4j</groupId>
           <artifactId>log4j</artifactId>
         </exclusion>
+        <exclusion>
+          <!-- we're using JUnit 4 -->
+          <groupId>org.junit.jupiter</groupId>
+          <artifactId>junit-jupiter</artifactId>
+        </exclusion>
       </exclusions>
     </dependency>
     <dependency>