Change MIXPANEL_TEST_ONLY to MIXPANEL_TEST_PRIORITY.

Mixpanel changed the meaning of the test flag in 2010, see 
http://blog.mixpanel.com/2010/04/22/a-way-to-ease-your-integration-with-mixpanel-while-were-working/

It doesn't make sense to keep calling our setting MIXPANEL_TEST_ONLY, but the
flag might still be useful, so keep it around as MIXPANEL_TEST_PRIORITY and
update the various documentation bits.