Shuffling code in test_multiple_inheritance.cpp to separate struct/class definitions from bindings code.

[rwgk]

Similar to #2875 but significantly simpler.

• Back-porting from smart_holder branch, to minimize diffs and the potential for merge conflicts. In particular, this PR is to help merge the smart_holder branch into the https://github.com/RobotLocomotion/pybind11 drake branch.
• Mostly code is just shuffled (see below for proof that nothing got lost).
• All code outside TEST_MODULE moved to the anonymous namespace. This isn't strictly necessary for the smart_holder work, but best practice.
• A few comments were duplicated for clarity, to appear both in the anonymous namespace and in TEST_MODULE. See diff below.

Changelog not needed.

Simple proof that nothing got lost:

cd pybind11/tests
git checkout master
cat test_multiple_inheritance.cpp | sed 's/^ *//' | grep -v '^$' | sort > zmaster
git checkout test_multiple_inheritance_non_interleaved
cat test_multiple_inheritance.cpp | sed 's/^ *//' | grep -v '^$' | sort > zpr
diff zmaster zpr
rm zmaster zpr

The full output from the diff command above:

30a31
> // but implement `struct`s and `class`es in the anonymous namespace above.
97a99,101
> } // namespace
> namespace {
> // Please do not interleave `struct` and `class` definitions with bindings code,
172a177
> // test_mi_base_return
175a181
> // test_mi_unaligned_base
180a187
> // test_multiple_inheritance_virtbase
183a191
> // This helps keeping the smart_holder branch in sync with master.

[EricCousineau-TRI]

Echoing @wjakob's sentiment in #2875 - looks straightforward, and it's always good to employ best practices!

[EricCousineau-TRI]

Ignoring stalled job on CI (see #2891); merging!