Remove the ability of `add_python_test()` to run without `TEST_PYTHON_EXE`

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.

Add `TEST_PYTHON_EXE` as advanced CMake option, and default to `${PYTHON_EXECUTABLE}` on macOS/Linux. The default value on Windows remains the Python from the installation directory.

Tests: On Linux, use Python from install dir

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 tries to find Python in the installation directory, which
will not be there on the first time CMake is run, so it also tries to
find it in the precompiled libraries. If that also fails, it defaults to
whichever Python executable CMake finds.

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