yang-system-test: Even without -v, output is full of DEBUG messages

The messages do not come from main(), but from parser (which uses slf4j as opposed to java.util.logging).

This probably affects the performance, and also possible error lines may be hard to locate visually.

An example output line:

1 [main] DEBUG org.opendaylight.yangtools.yang.parser.stmt.reactor.SourceSpecificContext - Source /home/jenkins/src/main/yang/experimental/odp/toaster-consumer-impl.yang started phase SOURCE_PRE_LINKAGE