testing: fix deadlock with t.Parallel in testing seed corpus

The c.startParallel channel on the testContext is stuck
in t.Parallel() because c.running starts at 1 for the main
fuzz parent test, and is causing a deadlock because it is
never released. It would normally be released by tRunner,
but needs to instead be released by fRunner instead for fuzz
tests.

Fixes #50217

Change-Id: I2d010e9adddfd8e8321ff2f9dd2e43daf46c128f
Reviewed-on: https://go-review.googlesource.com/c/go/+/374054


Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>