Speed up initial test creation in test_case_comprehensive.rb:

• Change generate_case_mapping_tests to
  generate_single_byte_case_mapping_tests
  (all encodings using this method happen to be single-byte)
• Change precalculation of codepoints by converting from
  specific encoding, not from Unicode. This reduces creation time.
  (See https://.com/ruby/ruby/pull/7425 for a related proposal
  and some additional comments.)