Speed up Unicode normalisation tests by a factor of 5

Speed up Unicode normalisation tests by a factor of 5

After this change, ucs-normalize-tests are still very slow but
somewhat less disastrously so (from 100 to 20 min on this machine).

* test/lisp/international/ucs-normalize-tests.el
(ucs-normalize-tests--normalization-equal-p)
(ucs-normalize-tests--normalization-chareq-p)
(ucs-normalize-tests--rule1-holds-p)
(ucs-normalize-tests--rule2-holds-p)
(ucs-normalize-tests--part1-rule2):
Run only over the Unicode code space.
Hoist `with-current-buffer` to reduce overhead.