os/signal: reset SIGURG in TestSignal

Accepting SIGURG signals could cause SIGURG to take up the entire
channel buffer.

Enhance the stability of test cases by:
1.  Stop accepting the SIGURG signal by adding  ‘Reset(sys call.SIGURG)’
2.  Close the c1 chan by adding ‘defer Stop(c1)’ (Another bug, NOT this bug)

Fixes #49724

Change-Id: I909a9993f0f6dd109c15e48a861683b87dfc4ab3
Reviewed-on: https://go-review.googlesource.com/c/go/+/366514


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>