Migrate aggregation and sampling system tests to Go.

As part of this we introduce "approval" test support.
The implementation is a little different to the one
in the Python system tests: the Python ones ignore
various dynamic fields when computing the diff; we
replace dynamic fields before comparing, so the
approved diffs don't change in unrelated ways every
time there is an expected change, such as a new field.

If we don't consume the response, the server
may (non-determinstically) log an error.