Re #7204. Move definition of varible closer to where it is used.

The PYTHON_EXE_WRAPPER variable, introduced under #6903, was being
defined up in the very top-level CMakeLists.txt. This meant that it
was not being set for a Framework only build of Mantid (i.e. when
cmake is pointed at Mantid/Framework).
Re #7204. Add the python-related testhelpers target to FrameworkTests.

Otherwise tests fail after only a 'make FrameworkTests'.
Revert "Re #7204. Move definition of varible closer to where it is used."

This reverts commit ed0163f88970d681096d54340ad11d7b1d6f444b.
Re #7204. Move variable definition so that Framework build sees it.
Merge branch 'bugfix/7204_framework_only_build' into develop