Use Stats.Enable in config instead of overloading Stats

Summary: Using "Stats" as both a boolean and as a section with children is not
only weird, it causes unintuitive behavior where overriding the section can
reset the boolean.

Reviewed By: @JoelMarcey, @markw65

Differential Revision: D1615419
Catchable fatals need %s replacement too

Summary: Following up on D1604585 where PHP 5.6.1 tests were imported, some good tests were moved to bad. Here is the first of a couple followup diffs to try to get them back.

We needed to have `%s` in `.expect` files for `Catchable fatal errors`

We have about 12 tests left to get back to good

Reviewed By: @paulbiss

Differential Revision: D1619651