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.

Author: liufengyun