fix flaky render test by dropping old spinners

[chris-laplante]

Before this change, the ticker_drop render test was occasionally
flaky.

Reproduce it by running this command:

while cargo test --all-features ticker_drop; do :; done

It will eventually fail with something like this:

running 1 test
test ticker_drop ... FAILED

failures:

---- ticker_drop stdout ----
thread 'ticker_drop' panicked at 'assertion failed: `(left == right)`
  left: `"⠁ doing stuff 0\n  doing stuff 1\n  doing stuff 2\n  doing stuff 3\n  doing stuff 4"`,
 right: `"  doing stuff 0\n  doing stuff 1\n  doing stuff 2\n  doing stuff 3\n  doing stuff 4"`', tests/render.rs:347:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Somehow the spinner is not correctly getting finished. Clearly there is
a subtle race condition somewhere but I haven't been able to track it
down.

[chris-laplante]

@djc I think this fixes what you encountered here: #458 (comment)

[djc]

If you reproduced this I think we should probably not paper over this in the test? At the very least we should file an issue about it.

[chris-laplante]

If you reproduced this I think we should probably not paper over this in the test? At the very least we should file an issue about it.

Sure, I'll file an issue. I'll close this PR then.