Fix #5: check test classes

[famod]

Fixes #5.

Note: The first commit is fixing the following bogus error when running mvn clean verify -f animal-sniffer-maven-plugin/ -Prun-its:

[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (mojo-enforcer-rules) @ animal-sniffer-maven-plugin ---
[WARNING] Rule 2: org.apache.maven.plugins.enforcer.RequirePluginVersions failed with message:
Some plugins are missing valid versions:(LATEST RELEASE SNAPSHOT are not allowed )
org.apache.maven.plugins:maven-compiler-plugin.         The version currently in use is 3.8.0
org.apache.maven.plugins:maven-surefire-plugin.         The version currently in use is 2.22.0
org.apache.maven.plugins:maven-jar-plugin.      The version currently in use is 3.0.2
org.apache.maven.plugins:maven-clean-plugin.    The version currently in use is 3.0.0
org.apache.maven.plugins:maven-checkstyle-plugin.       The version currently in use is 2.16
org.apache.maven.plugins:maven-install-plugin.  The version currently in use is 2.5.2
org.apache.maven.plugins:maven-site-plugin.     The version currently in use is 3.7.1
org.apache.maven.plugins:maven-invoker-plugin.  The version currently in use is 3.2.1
org.apache.maven.plugins:maven-resources-plugin.        The version currently in use is 3.0.2
org.apache.maven.plugins:maven-deploy-plugin.   The version currently in use is 2.8.2
org.apache.maven.plugins:maven-enforcer-plugin.         The version currently in use is 3.0.0-M2
Best Practice is to always define plugin versions!

[famod]

There are some very strange artifact download problems in some invoker tests (ReasonPhrase:Forbidden) in 3 travis builds, e.g.:

[INFO] Downloaded from central: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-java/0.9.2/plexus-java-0.9.2.jar (31 kB at 693 kB/s)
[INFO] Downloaded from central: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-compiler-api/2.8.2/plexus-compiler-api-2.8.2.jar (26 kB at 330 kB/s)
[INFO] Downloaded from central: https://repo.maven.apache.org/maven2/org/ow2/asm/asm/6.0_BETA/asm-6.0_BETA.jar (56 kB at 533 kB/s)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.974 s
[INFO] Finished at: 2020-03-20T19:10:48Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile (default-compile) on project real-test: Execution default-compile of goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile failed: Plugin org.apache.maven.plugins:maven-compiler-plugin:3.7.0 or one of its dependencies could not be resolved: The following artifacts could not be resolved: org.codehaus.plexus:plexus-compiler-manager:jar:2.8.2, org.codehaus.plexus:plexus-compiler-javac:jar:2.8.2: Could not transfer artifact org.codehaus.plexus:plexus-compiler-manager:jar:2.8.2 from/to central (https://repo.maven.apache.org/maven2): Access denied to: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-compiler-manager/2.8.2/plexus-compiler-manager-2.8.2.jar , ReasonPhrase:Forbidden. -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile (default-compile) on project real-test: Execution default-compile of goal org.apache.maven.plugins:maven-compiler-plugin:3.7.0:compile failed: Plugin org.apache.maven.plugins:maven-compiler-plugin:3.7.0 or one of its dependencies could not be resolved: The following artifacts could not be resolved: org.codehaus.plexus:plexus-compiler-manager:jar:2.8.2, org.codehaus.plexus:plexus-compiler-javac:jar:2.8.2: Could not transfer artifact org.codehaus.plexus:plexus-compiler-manager:jar:2.8.2 from/to central (https://repo.maven.apache.org/maven2): Access denied to: https://repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-compiler-manager/2.8.2/plexus-compiler-manager-2.8.2.jar , ReasonPhrase:Forbidden.

[famod]

@olamy ping