libutils: Add more tests for Unicode

This CL added additional tests for converting between utf16 and utf8,
specifically tests that:

  1. check utf16_to_utf8_length() returns 0 if input is an
     empty UTF16 string

  2. check utf16_to_utf8_length() returns 1 if input is a
     single ASCII character UTF16 string

  3. check utf16_to_utf8_length() returns 3 if input is a
     single UTF-16 character between U+0800 - U+FFFF

  4. check utf16_to_utf8_length() returns 4 if input has
     a surrogate pair

  5. check unpaired UTF-16 surrogate is handled correctly
     (skipped)

  6. check utf16_to_utf8_length(0 handles invalid surrogate
     case correctly, by skipping the first but handling the
     rest correctly

  7. check a normal string with a mix of 1/2/3/4-byte UTF8
     characters is correctly converted by utf16_to_utf8()

  8. check conversion from invalid utf8 sequence with invalid
     leading byte and/or invalid trailing byte(s) should still
     work and not crash

Change-Id: If68e514af0e84ddebf5900b2e140e76ba4f44553