runtime: fix TestAbort on Plan 9
runtime: fix TestAbort on Plan 9

Since CL 122515, TestAbort is failing on Plan 9
because there is no SIGTRAP signal on Plan 9,
but a note containing the "sys: breakpoint" string.

This change fixes the TestAbort test by handling
the Plan 9 case.

Fixes #26265.

Change-Id: I2fae00130bcee1cf946d8cc9d147a77f951be390
Reviewed-on: https://go-review.googlesource.com/122464
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,josharian/go,CAFxX/go,josharian/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,golang/go,akutz/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,josharian/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go,golang/go,nawawi/go,josharian/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,akutz/go