runtime: skip TestStackSwitchCallback on ios

runtime: skip TestStackSwitchCallback on ios

getcontext is unsupported on ios, returning ENOTSUP.

Fixes #62700.

Change-Id: Icc6c45fbd574886766d5289e2fbdbd62981d77eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/529216
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>