internal/fuzz: temporarily work around test failures after dev.fuzz merge
- Skip test_fuzz_cache and test_fuzz_seed_corpus on 386.
- Skip worker benchmarks when race mode is enabled.
- Stub coverage function on platforms we haven't tested yet. It's
causing package initialization to panic on aix/ppc64.
For #48504
Change-Id: I79318b52b11a33fca66476b5050445d07422ef36
Reviewed-on: https://go-review.googlesource.com/c/go/+/351117
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Katie Hockman <katie@golang.org>