Finally figured it out.
I had Logback on the classpath, so to disable it I had to set the log level in Logback instead of on the logger instance because the setLogLevel() method in Slf4jLogger no-ops.
I basically did before running my Liquibase test command.
val logger = LoggerFactory.getLogger("liquibase")
(logger as ch.qos.logback.classic.Logger).level = Level.ERROR