817 automatically perform module purge before building tests

[hwikle-lanl]

Code review checklist:

• Code is generally sensical and well commented
• Variable/function names all telegraph their purpose and contents
• Functions/classes have useful doc strings
• Function arguments are typed
• Added/modified unit tests to cover changes.
• New features have documentation added to the docs.
• New features and backwards compatibility breaks are noted in the RELEASE.md

[Paul-Ferrell]

This should be an option to build/run, but the default should be 'true'.

[Paul-Ferrell]

I saw that you added a commit that module purges only when modules are specified. It should be always (if enabled), with checks for whether a module system is present (IE - does the module alias exist?