Applying 'java-test-fixtures' plugin caused 'No mutations found'
from Pitest.

The plugin changes test runtime classpath:
tested code is included as a JAR instead of as a directory
(see gradle/gradle#11696). It seems that Pitest ignores correct
directories being passed using --mutableCodePaths and insists on
having those directories also passed in --classPath.

Also change functional tests: runTasksSuccessfully() fails the test
without giving any context, so it should be avoided.