Set FORCE_BUILD_SMP in the python, not in the XML

[jgfouca]

This way, tests like ERP don't have to turn this on unless it's really needed.

Description

There was confusion as to why an ERP test was turning on threaded builds even though both case1 and case2 had NTHRDS=1.

Checklist

• My code follows the style guidlines of this proejct (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that excerise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation