Fuzztest: exclude instrunentation from TSAN.

TSAN reports a data race in checking whether instrumentation is enabled.
Some searching in the bug tracker suggests that this is intentionally
not protected by a mutex for performance reasons, so add it to TSAN
exclusion list.

Bug: 40286211
Change-Id: Icb17b57468b8589f5148d84ba142e7932f3bca21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6063331
Reviewed-by: Ali Hijazi <ahijazi@chromium.org>
Auto-Submit: Adrian Taylor <adetaylor@chromium.org>
Commit-Queue: Ali Hijazi <ahijazi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1391551}

build/sanitizers/tsan_suppressions.cc

@@ -97,6 +97,10 @@ char kTSanDefaultSuppressions[] =
     // and the race is therefore not present in released builds.
     "race:crash_reporter::*::OutputCrashKeysToStream\n"
 
+    // Tracing within fuzztest instrumentation, likely intentionally not
+    // synchronized for performance.
+    "race:SetIsTracing\n"
+
     // End of suppressions.
     ;  // Please keep this semicolon.