Add char static casts to crypto_utils_test.cc
Add char static casts to crypto_utils_test.cc

When rolling quiche to Chromium, the compiler complains with narrowing
conversion error (-Wc++11-narrowing). Hence, this change adds static
casts to convert to char to silence this issue.

PiperOrigin-RevId: 425339124

google/quiche,google/quiche,google/quiche,google/quiche