By default tests are executed with the same Java Gradle is using internally.
To run tests against a different Java version, define a property called runtime.java.home or define an environment variable RUNTIME_JAVA_HOME pointing at the JDK installation folder.
If property is being used, it can be a system property (-D…) or a project property (-P…).
Example:
gradlew test -p solr/test-framework --tests TestLogLevelAnnotations -Dtests.verbose=true -Druntime.java.home=/jvms/jdk14
Note that an alternative JVM can also be made the "default" setting by adding it to (project-local) gradle.properties.