Use set_up_inc for several unit tests

Use set_up_inc for several unit tests

Use set_up_inc when require.pl is loaded
move plan outside of BEGIN block
when no tests are run at BEGIN time.

Using set_up_inc allow to run these tests under minitest
but also compile them using B::C.

This also has the advantage to use a single
control point for @INC setup.

Note: some tests cannot use 'require test.pl',
unshfit is then used for them.