fuzztest: Include -Wno-unused-result flag to internal fuzztest config

The code coverage builder has been failing to build since February. Due
to https://crrev.com/c/7557779, `warn_unused_result` was added to the
`write` function in unistd.h. This led to failing builds for the
linux-centipede-fuzz-coverage builder. This CL adds -Wno-unused-result
for internal fuzzers to suppress those errors, since fuzztest is a third
party dependency.

Cq-Include-Trybots: luci.chromium.try:linux-centipede-asan-rel
Bug: 500385597
Change-Id: I2dbd224acc4d3beffe90bb2395c275843ce3d7b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7747333
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Commit-Queue: Antonio Alphonse <alphonsea@google.com>
Cr-Commit-Position: refs/heads/main@{#1613104}
NOKEYCHECK=True
GitOrigin-RevId: d9e293283987f15e5e550086ab8bcb31b812ac5c