[perl #116971] avoid feeding utils run by utils.t a bad @INC

the C<< use TestInit qw(T) >> both changes to the build tree root
*and* initializes $ENV{PERL5LIB} to "lib", so there's no need for
runperl() to supply -I.. when testing each tool.
[perl #116971] avoid feeding manisort a bad @INC

the C<< use TestInit qw(T) >> in manifest.t both changes to the
build tree *and* initializes $ENV{PERL5LIB} to "lib", so there's
no need fo runperl() to supply (an incorrect) -I.. when running
manisort.
use the more portable File::Spec->devnull for the stderr redirect

this allows pending-author.t to work on MSWin32