tests: Add infrastructure to run Python unit tests

- Add UHD_ADD_PYTEST() CMake macro
- Add CMake code to tests/CMakeLists.txt to auto-run all registered
  Python unit tests
- Add a token unit test (it replicates parts of ranges_test.cpp)

The way Python-based unit tests are implemented in UHD is that they can
import uhd, and then operate on the module as usual.

Writing unit tests in Python instead of C++ can have multiple
advantages:
- If they test PyBind-wrapped C++ code, they can test both the binding
  and the underlying C++ code at once
- Writing unit tests in Python may be more concise

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