os/signal: reset SIGURG in TestSignal
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

nawawi/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,golang/go