Use newer / more common test framework

[ethanfrey]

nyc/ava is a relic of a previous dev, but no one uses it.

We cannot use watch tests anyway with this codebase, as they run a minute or so full relayer tests.

We could migrate to something more common. And ideally that gives better stack traces on errors

[webmaster128]

jest is very common (and has test watching). In CosmJS we still use Jasmine because in contrast to jest it runs well in the browser.

Both share the same syntax.

[ethanfrey]

We have no need to watching.
I am not sure if we care much about the browser use case, but I consider it a nice to have

Either jasmine or jest seem reasonable. Happy for a pr to either one.