Re-write many skip decorators to use shared code.

This should be no functional change, just a refactoring of the
skip decorators to all centralize on a single function,
`skipTestIfFn` that does all the logic.  This allows easier
maintenance of the decorators and also centralizes all the
hard-to-understand logic in one place.

Reviewed by: Pavel Labath
Differential Revision: http://reviews.llvm.org/D16741

llvm-svn: 259543