Make tests more predictable by removing setState callbacks

[kumar303]

I don't think our tests should rely on setState() being synchronous as it's unclear that this will always work in enzyme as its libraries evolve and get upgraded.

We have a few problems in the test suite related to this but first let's try not to rely on setState callbacks.

Here are two places that (hopefully) can be fixed with different approaches:

• Avoid fetching suggestions all the time addons-frontend#3058 (review)
• ShowMoreCard should expand if updated content changes addons-frontend#3057 (review)

[willdurand]

should be fixed now.