Make `unicode_data` tests normal

Instead of generating a standalone executable to test `unicode_data`,
generate normal tests in `coretests`. This ensures tests are always
generated, and will be run as part of the normal testsuite.

Also change the generated tests to loop over lookup tables, rather than
generating a separate `assert_eq!()` statement for every codepoint. The
old approach produced a massive (20,000 lines plus) file which took
minutes to compile!