[dev.fuzz] testing: read corpus from testdata/corpus for each target

[dev.fuzz] testing: read corpus from testdata/corpus for each target

This change also includes a small cleanup of the run()
function and additional tests for error conditions
in fuzz targets.

Change-Id: I2b7722b25a0d071182a84f1dc4b92e82a7ea34d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/256978
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Katie Hockman <katie@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>