[WARNING] Auxclasspath entry file:/home/travis/build/checkstyle/checkstyle/target/test-classes doesn't exist, ignoring it

Sounds suspicious, we need to make sure that tests are not ignored

Confirmed. The command line should be
mvn -e clean test-compile pmd:check spotbugs:check

But this command fails with
[ERROR] The field com.puppycrawl.tools.checkstyle.DefaultConfiguration.$jacocoData is transient but isn't set by deserialization [com.puppycrawl.tools.checkstyle.DefaultConfiguration] In DefaultConfiguration.java SE_TRANSIENT_FIELD_NOT_RESTORED

There should be exception for the field injected by JaCoCo.

Originally posted by @pbludov in #8254 (comment)