[perl #43314] [TODO] config/auto/msvc.pm: Write unit tests

[James Keenan via RT]

Internals of this step's runstep() were refactored into internal
subroutines, which were then unit-tested in additional test files.  I
have narrowed the scope of the SKIP block in 113-auto_msvc-01.t to
simply the test of the runstep() method.  Test coverage is high. 
Resolving ticket.