Add test case for `scm_take_u8vector ()'.

Add test case for `scm_take_u8vector ()'.

This is a followup to commit d7e7a02a6251c8ed4f76933d9d30baeee3f599c0
("Fix leaky behavior of `scm_take_TAGvector ()'.") and a reminder that
the uniform vector implementation can't do away with the cell->buffer
indirection.

* test-suite/standalone/Makefile.am (test_scm_take_u8vector_SOURCES,
  test_scm_take_u8vector_CFLAGS, test_scm_take_u8vector_LDADD): New.
  (check_PROGRAMS, TESTS): Add `test-scm-take-u8vector'.

* test-suite/standalone/test-scm-take-u8vector.c: New file.