This updates tests for `AVA` 19.0.0+.
`AVA` 0.18.0 has a breaking change which changes the current working directory
for tests. As a result, we need to change 'resources' -> 'test/resources' (and
similar path changes).
`AVA` 0.19.0 has a breaking change that all tests must run at least one assert.
This breaking change was already resolved by #746, so no change was necessary in
this PR.
This updates to `AVA` 0.21.0, since there are no other breaking changes.