Update t/harness to reflect behaviour changes in TAP::Harness 3.16

Update t/harness to reflect behaviour changes in TAP::Harness 3.16
Explicitly add -I../lib to the switches, rather than rely on TAP::Harness or
Test::Harness picking -I../lib up from $ENV{PERL5LIB}. (Test::Harness hasn't
changed, but TAP::Harness has). Test::Harness does, however, spot that we've
altered our own @INC, and passes this on to tests. So there is no need to set
$ENV{PERL5LIB} to effect changes to @INC in tests. So don't set $ENV{PERL5LIB}
to -I../lib. Instead set it to '', to avoid contamination from our environment.