gddo-server: fix requestEvent reference in pkgsite_test.go

gddoEvent was incorrectly renamed to requestEvent in CL 274860, which is
now fixed.

Change-Id: I16b2cb483994fbe3a2adddf800fcf712b31c54b4
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/279034
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>