runtime: avoid loop variable capture in test

In TestSegv, the t.Run closure captures the loop variable 'test'. Since
the subtest calls t.Parallel, the parent test is allowed to keep
running, changing the loop variable and thus changing the value of
'test' in the subtest.

Change-Id: I021ddc50304de08a341e6ffe486aa54e573d3b94
Reviewed-on: https://go-review.googlesource.com/c/go/+/339911


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>