bindings: python: don't install test-specific C extension binaries

bindings: python: don't install test-specific C extension binaries

We want to ship the source code for C extensions used by the test suite
but not install the built shared objects or put them into the bdist.
Extend the build_ext command to delete the tests from the build directory
right after the extensions have been built and - possibly - installed
into the source tree (if --inplace is set).

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>