cmd/link: fix TestDWARFiOS

Use GOOS=ios.

Run "xcodebuild -showsdks" correctly.

Wrap testDWARF in subtests, as it calls t.Parallel, otherwise
the two calls of testDWARF will cause t.Parallel being called
twice, which panics.

Updates #38485.

Change-Id: I614c8daa99c83cbfd05a4cfa041968d49279f16a
Reviewed-on: https://go-review.googlesource.com/c/go/+/263639


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>