Disable creation of Log4J JMX beans in Gradle test conventions

[sdavids]

Would it make sense to disable the creation of Log4J JMX beans?

Steps to reproduce

junit5/gradle/plugins/common/src/main/kotlin/junitbuild.testing-conventions.gradle.kts

Line 43 in ccd24b3

systemProperty("java.util.logging.manager", "org.apache.logging.log4j.jul.LogManager")

⇓

systemProperty("java.util.logging.manager", "org.apache.logging.log4j.jul.LogManager")
systemProperty("log4j2.disableJmx", "true")

Context

To disable JMX completely, and prevent these MBeans from being created, specify system property log4j2.disableJmx to true when you start the Java VM.

https://logging.apache.org/log4j/2.x/manual/jmx.html#enabling-jmx

[sbrannen]

As long as we are not relying on their presence, it sounds reasonable to disable the JMX beans.

@marcphilipp and @sormuras, do you foresee any issue with disabling them?

[sormuras]

No. Our tests don't rely on JMX to configure Log4J at runtime.

PR also looks good to me.