Blender Git Loki

Tests: use explicit Python to run unit tests

CentOS on the buildbot still runs Python 3.6, which is also used for the
unit tests. This means that the tests can't use language features that
are available to Blender itself. And testing with a different version of
Python than will be used by the actual code seems like a bad idea to me.

This commit adds `TEST_PYTHON_EXECUTABLE` as advanced CMake option. This
will allow us to set a specific Python executable when we need it. When
not set, a platform-specific default will be used:

- On Windows, the `python?.exe` from the installation directory. This is
just like before this patch, except that this patch adds the
overridability.
- On macOS/Linux, the `${PYTHON_EXECUTABLE}` as found by CMake.

Every platform should now have a value (configured by the user or
detected by CMake) for `TEST_PYTHON_EXE`, so there is no need to allow
running without. This also removes the need to have some Python files
marked as executable.

If `TEST_PYTHON_EXE` is not user-configured, and thus the above default
is used, a status message is logged by CMake. I've seen this a lot in
other projects, and I like that it shows which values are auto-detected.
However, it's not common in Blender, so if we want we can either remove
it now, or remove it after the buildbot has been set up correctly.

Differential Revision: https://developer.blender.org/D7395

Reviewed by: campbellbarton, mont29, sergey