These wasm tests weren't passing in GitHub Actions and also weren't
passing on my laptop. I'm not sure why, I think there are a few race
conditions that are going on.
This commit attempts to fix this at least to a degree:
- The context deadline is increased from 5 seconds to 10 seconds.
- The tests are not running in parallel anymore.
- Some `Sleep` calls were removed, they do not appear to be necessary
(and if they were, sleeping is the wrong solution to solve race
conditions).
Overall the tests are taking a few seconds more, but on the other hand
they seem to be passing more reliable. At least for me, on my laptop
(and hopefully also in CI).
