[Cashtab] Use correct jest syntax in unit tests

Summary:
Found some Cashtab tests using assert instead of jest syntax in unit tests. These can still ... work. Sort of. However the point of using jest is to better test the react environment vs node.

Can see there was one issue in the cashMethods.test.js where the wrong type was being tested and asset did not catch it.

Test Plan:
npm test
grep -r assert src/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15056