This commit removes all occurrences of `--unstable` flag
from all the tests that are run in CI.
Turns out none of the tests actually required that flag
anymore.
It's not clear to me how these tests worked correctly on CI,
but they were failing hard locally because of two problems:
- missing env var that tests URL for fake npm registry
- trying to run a directory that contains native Node.js tests that
require a special harness
