go/ssa: skip the "constraints" package in TestStdlib
go/ssa: skip the "constraints" package in TestStdlib

go/ssa doesn't yet support generics, so the "constraints" package causes
this test to fail.

Updates golang/go#48595

Change-Id: I11c5a944bffba0c0398cd48ffca5d80fbede4332
Reviewed-on: https://go-review.googlesource.com/c/tools/+/352051
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools