Fix spurious failure in CI

The failure can be seen below (thanks to @griggt):

https://github.com/lampepfl/dotty/runs/1732363505?check_suite_focus=true

On my local machine, I can reproduce the error both under Java 14 and
Java 8. The error stacktrace is in TreeChecker.

By reducing the size of the enum, we avoid such instability in test.

[test_java8]

Author: liufengyun