testing: only snapshot coverage during fuzzing

testing: only snapshot coverage during fuzzing

Only snapshot/reset coverage counters when we are actually fuzzing.
This prevents a race when running corpus/seed values during the testing
phase.

Fixes #50488

Change-Id: I7dd5a0353a296c0b13eede29ad9af7c78814fa2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/376554
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>