This reworks query comparison in compare_queries() and

assertQueries() to track and compare nested subqueries. Queries now

support a subqueries field, which follows the same format of the main

expected queries list, and this can be nested as deeply as required for

the queries being executed.
Any subquery inside of a where filter (via Subquery() or Exists())

will be transformed into a Q(__SubqueryType__subquery__=index) query,

since this allows for direct comparison and equates to an index within

the nested query comparison list.
The recent support for inner_query has been removed in favor of this

new support. Support is opt-in currently for assertQueries(), to

maintain backwards-compatibility, but will be enabled by default in

Djblets 5.
The normalization and formatting support has been enhanced to support

subqueries and F-expressions, and to display the way the code would be

written in Python with Q(...), F(...), etc. This makes it far easier

to compare and update queries.

Unit tests pass.
Tested this with in-progress code in Review Board.