Add fuzzer to try ToUpper/ToLower

Also change the fromUTF32 function to make the string distrbute
more evenly in the valid range of UTF32 but still have a small
% of invalid UTF32.

Bug: 1078274
Change-Id: I316cb857b7304398d0f660ad09f282be1f23d5b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/2228417
Reviewed-by: Jungshik Shin <jshin@chromium.org>
3 files changed