cmd/link: skip TestSizes on Plan 9
cmd/link: skip TestSizes on Plan 9

TestSizes has been added in CL 55551. This test is
failing on Plan 9 because executables don't have
a DWARF symbol table.

Fixes #21453.

Change-Id: I560611b49aea5417e8c5ac0cec6c7882bd9f8335
Reviewed-on: https://go-review.googlesource.com/55692
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>

codestation/go,golang/go,cloudflare/go,golang/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,CAFxX/go,polyverse-security/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,nawawi/go,golang/go,josharian/go,cloudflare/go,golang/go,polyverse-security/go,polyverse-security/go,codestation/go,josharian/go,christopher-henderson/Go,josharian/go,codestation/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,josharian/go,nawawi/go,nawawi/go,polyverse-security/go,akutz/go,Cofyc/go,Cofyc/go,cloudflare/go,codestation/go,Cofyc/go,golang/go,Cofyc/go,codestation/go,CAFxX/go,golang/go,akutz/go,golang/go,akutz/go,Cofyc/go,polyverse-security/go,codestation/go,codestation/go,josharian/go,nawawi/go,nawawi/go,polyverse-security/go,Cofyc/go,josharian/go,polyverse-security/go,cloudflare/go,CAFxX/go,akutz/go,Cofyc/go,CAFxX/go,akutz/go,codestation/go,polyverse-security/go,Cofyc/go