Have unit test reference proper checker-qual version.

If the `checker-qual` version referenced by the unit tests doesn't match the version declared in `pom.xml`, then `JavacJSR308CompilerTest` will fail unless (through chance) the user has previously downloaded the other version as well.

Following this change, the following command should complete successfully: `mvn clean install -Dmaven.repo.local=/tmp/empty-directory`.