This commit adds tests for the `jceks` package against the
installed JDK's `keytool` CLI. The tests ensure that JCEKS files
created by `keytool` can be read by the package, and vice versa.
These tests are disabled by default and are enabled with the
`-jceks.keytool-tests=true` flag. This commit configures GitHub
Actions to test the package against a wide variety of JDKs.
This commit also restricts GitHub Actions permissions.
