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This adds VS 2017 to the build matrix, plus various other small
appveyor build changes:

- conda version bumped from 3.5 to 3.6
- build newer versions/architectures/python first (i.e. VS2017/x64/3.6
  is the first build, VS2015/x86/2.7 is the last)
- stop building after a job failure: often a build failure in one
  occurs everywhere; this just stops processing jobs (freeing them up
  for other PRs) if an error is hit.

Annoyingly, appveyor doesn't allow excluding tests: i.e. the test matrix
is always dense (appveyor issue 386), so for now we'll just run
everything.  (Once appveyor issue 386 is resolved, we can come back and
cut this down to 4-5 builds).

Add a BUILD_INTERFACE and a pybind11::pybind11 alias for the interface
library to match the installed target.

Add new cmake tests for add_subdirectory and consolidates the
.cpp and .py files needed for the cmake build tests:

Before:
tests
|-- test_installed_module
|   |-- CMakeLists.txt
|   |-- main.cpp
|   \-- test.py
\-- test_installed_target
    |-- CMakeLists.txt
    |-- main.cpp
    \-- test.py

After:
tests
\-- test_cmake_build
    |-- installed_module/CMakeLists.txt
    |-- installed_target/CMakeLists.txt
    |-- subdirectory_module/CMakeLists.txt
    |-- subdirectory_target/CMakeLists.txt
    |-- main.cpp
    \-- test.py

With this change both C++ and Python write to sys.stdout which resolves
the capture issues noted in #351. Therefore, the related workarounds are
removed.

pytest can capture test output both globally (controlled by the cmd line
flag --capture) or locally (`capsys` and `capfd` fixtures). Enabling both
methods at the same time causes problems on Windows: test output is not
captured sometimes, resulting in test failure. This happens seemingly at
random.

This workaround disables global output capture ("-s", i.e. "--capture=no")
leaving only the local capture fixtures. As a side-effect test output on
AppVeyor CI is a little messy, but this will have to do until a better
solution is found.

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.