qmake: break out testFunc_cache() from evaluateBuiltinConditional()

It's 160ish lines and adequately isolated.  Still, it *might* be
contributing to compilers being slow (unlikely though that seems) so
seemed worth tidying up anyway.

Task-number: QTQAINFRA-2117
Change-Id: I9e55e677552c273fdf3480ccefc229fd6fd2b66a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
(cherry picked from qtbase/01eab308a3d6dbfc9739cc822fea064180350e18)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>