Moved tests for render shortcuts to their own app.
Avoided rewrapping Contexts in render_to_response.

This change preserves backwards-compatibility for a very common misuse
of render_to_response which even occurred in the official documentation.

It fixes that misuse wherever it happened in the code base and docs.

Context.__init__ is documented as accepting a dict and nothing else.
Since Context is dict-like, Context(Context({})) could work to some
extent. However, things get complicated with RequestContext and that
gets in the way of refactoring the template engine. This is the real
rationale for this change.
Silenced a flake8 warning.

../tests/shortcuts/views.py:45:1: E302 expected 2 blank lines, found 1
Fix another flake8 warning.

I don't agree with flake8 here about the right indentation, but as long as
we're using it, we should stick to it. I don't want to disable its hanging
indent checks just because of this case.
Fixed #23892 -- Made deconstructible classes forwards compatible
Changed test_runner imports to name objects imported from test.runner.
Improved line breaking in runtests docs.
Fixed #23742 -- Added an option to reverse tests order.

This is useful for debugging side-effects affecting tests that
are usually executed before a given test. Full suite and pair
tests sort cases more or less deterministically, thus some test
cross-dependencies are easier to reveal by reversing the order.

Thanks Preston Timmons for the reviews.