Improve the testing methods for invoking API requests.

Review Request #10397 — Created Jan. 30, 2019 and updated — Latest diff uploaded


This is a rewrite of the various API invocation methods for unit tests
that simplifies them, moving all common logic to a single function
(api_call, formerly _api_func_wrapper). The various wrapper methods
now just invoke this method with some default arguments. Subclasses can
override api_call to add further logic, and the other methods will
benefit from it.

These also now allow for specifying the number of SQL queries that an
API handler was expected to have run. This is done by passing
expected_num_queries= to any of the API invocation methods.

The new methods are backwards-compatible with the old ones, so long as
the various keyword arguments (anything past path and query) weren't
passed as positional arguments. However, it does rename the query
argument to data, which makes more sense for non-GET requests. If
passed as a positional argument, nothing changes, but if passed as a
keyword argument, a warning will be emitted and the argument will be

Djblets and Review Board unit tests pass.