[Merge] minitest fixes

If miniperl is sufficiently broken that make_ext.pl can’t run, it’s
useful to run minitest and debug the individual tests to find out what
is broken.

I was doing just that and ran into one false positive after another,
which greatly reduces the usefulness of minitest.

This branch fixes up the tests to make sure miniperl can find the nec-
essary modules, skipping tests where that is not possible.

test.pl has two new functions:

- set_up_inc, which clobbers @INC under real perl, but unshifts
  under miniperl

- skip_all_without_unicode_tables, which skips the whole test script
  if Unicode tables have not been built yet, but only under miniperl;
  real perl is not allowed to skip the tests because of that, since
  the absence of Unicode tables indicates a broken perl.