cmd/internal/obj/x86: avoid os.Chdir in issue19518_test.go
cmd/internal/obj/x86: avoid os.Chdir in issue19518_test.go

Chdir leaves the test in the wrong working directory if objdumpOutput
calls t.Fatalf (or panics), and it isn't necessary here anyway. Set
the Dir field on the commands instead.

Change-Id: I9f0eb0d4f8d15043f1e13472126ca1a1ce4b7cb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/167081
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

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