Downgrade to surefire 2.19.1

The combination of Java 11, the junit version, the surefire plugin version and
the plugin config `forkCount` and `reuseForks` caused a variety of
failures when running the tests, most frequently a ClassNotFoundException like this:

ClassNotFoundException: org.apache.maven.plugin.surefire.StartupReportConfiguration

It is not entirely clear what the correct way to fix this but it occurs
even when you use version 3.0.0-M3 of the surefire plugin, which is the
newest released version as of writing this. Perhaps also upgrading to
junit 5 might resolve this, but that seemed a little beyond the scope of
the PR.

The least invasive choice was to downgrade the surefire plugin version.

junit-team/junit5#1367
https://jira.apache.org/jira/browse/SUREFIRE-1520
https://issues.apache.org/jira/browse/SUREFIRE-1534
https://stackoverflow.com/questions/53437819/maven-surefire-and-jdk-11

pom.xml

@@ -258,7 +258,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-surefire-plugin</artifactId>
-                <version>3.0.0-M3</version>
+                <version>2.19.1</version>
                 <configuration>
                     <argLine>-Xmx2G</argLine>
                     <argLine>-Dfile.encoding=UTF-8</argLine>