Make `org.junit.jupiter:junit-jupiter` an optional test dependency again

[linghengqian]

• In Bump GraalVM Native Build Tools to 0.11.0 #34 , due to a bug in GraalVM Native Build Tools 0.11.0, org.junit.jupiter:junit-jupiter became an unavoidable test dependency.
• This is counterintuitive. Further investigation awaits Group annotations by dependency and print a warning if the dependency is missing graalvm/native-build-tools#752 , maven plugin: IllegalArgumentException while collecting optional dependency graalvm/native-build-tools#756 , and JupiterConfigProvider fails with ClassNotFoundExecption graalvm/native-build-tools#747 .

[linghengqian]

• I'm tracking this on the current issue.