AssertJ has more powerful test assertions and does not run the risk of
interfering with the classpath of main code, such as guava does. This
removes guava from test and example code and adjusts using AssertJ in
some cases.
JUnit Rules, such as MockWebServerRule, reduce boilerplate setup present
in our tests. By migrating off TestNG, and onto rules, our tests become
more maintainable as JUnit is well understood.