fix unit test by waiting for leader election

[matthiashanel]

Signed-off-by: Matthias Hanel [email protected]

• [ ] Link to issue, e.g. Resolves #NNN
• [ ] Documentation added (if applicable)
• [ ] Tests added
• [ ] Branch rebased on top of current main (git pull --rebase origin main)
• [ ] Changes squashed to a single commit (described here)
• [ ] Build is green in Travis CI
• [ ] You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

Resolves #

Changes proposed in this pull request:

/cc @nats-io/core