all: update terminology for fuzzing

This change doesn't modify any functionality.
It also doesn't update all of the comments and
variable names of the internal code, but everything
user facing should be correct.

Updates #49185

Change-Id: Ia8b2c94b89ba45897c4085ea0c17a3d8896f7ec7
Reviewed-on: https://go-review.googlesource.com/c/go/+/362794


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>