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 <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>