[dev.fuzz] cmd/go: in 'go test' don't allow multiple packages with -fuzz

Until we have a system for managing load across multiple fuzz targets
in multiple test executables, we'll only support fuzzing one target in
one package at a time. Users can still run multiple 'go test -fuzz'
commands concurrently, but this may overwhelm some systems unless
-parallel and -p are set carefully.

For #46312

Change-Id: If84c58d1b3e60498ce955eae5ad4d52100dbd4b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/350156


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>