Make all tests run on Java 16 via '-add-opens' JVM option

Due to JEP 260 (Encapsulate Most Internal APIs), aspect weaving on
Java 16 now requires '--add-opens java.base/java.lang=ALL-UNNAMED' on
the command line. Otherwise there will be illegal access exceptions for
some internal API calls AspectJ needs, most prominently when trying to
define classes in other packages or modules.

If in the future we find a way to access the functionality involved
again without adding CLI parameters, this tag helps identify the commit
which should be reverted.