bindings: python: standalone build tooling for tests

bindings: python: standalone build tooling for tests

Move extension definitions and tooling for building tests into
`build_tests.py` and update Makefile.am to call it with appropriate path
prefixes.

`build_tests.py` will perform a standalone build of the text extensions,
keeping any build noise in a temporary directory and copying the final
built modules automatically out to `tests/gpiosim` and `tests/procname`.

Add "python-tests-run" to Makefile.am so it's clear how to run the tests.

Add .so object files generated by build_test.py to Makefile.am's
clean-local.

Signed-off-by: Phil Howard <phil@gadgetoid.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>