Allows STR_UP_4 to satisfy MUT_TOUGH prereq, as 3 already did

This matters most for beast threshold which was recently given access
to MUT_TOUGH, but doesn't qualify for any of the non-strength
prereqs. Since the goal here seems to be "already has some kind of
post-threshold ability to take punishment" it seems to fit.

Previously, if you had THRESH_BEAST and STR_UP_4 you would need to
roll MUT_TOUGH, make a 1 in 4 roll for it to pick STR_UP_3 as the
target prereq (downgrading your STR_UP_4), and then on your next
mutation make a 1 in 2 chance to roll MUT_TOUGH again before rolling
STR_UP_4 again even though MUT_TOUGH is in-category for you.

I suspect this never came up before because beast wasn't eligible for
MUT_TOUGH until June, and ursine would almost always have already
gained LARGE_OK or HUGE_OK before getting STR_UP_4.
Also added a leads_to from STR_UP_4 to MUT_TOUGH to match STR_UP_3
Merge pull request #8865 from rluddy/tough-stuff

Allows STR_UP_4 to satisfy MUT_TOUGH prereq, as 3 already did