runtime: don't call t.Parallel in TestCgoSignalDeadlock

runtime: don't call t.Parallel in TestCgoSignalDeadlock

It seems that when too much other code is running on the system,
the testprogcgo code can overrun its timeouts.

Updates #18598.

Not marking the issue as fixed until it doesn't recur for some time.

Change-Id: Ieaf106b41986fdda76b1d027bb9d5e3fb805cc3b
Reviewed-on: https://go-review.googlesource.com/48233
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>