crypto/tls: skip part of the test based on GOOS instead of GOARCH

This allows to skip the last part of the test under GopherJS as well as
WebAssembly, since GopherJS shares GOOS=js with wasm.

Change-Id: I41adad788043c1863b23eb2a6da9bc9aa2833092
GitHub-Last-Rev: d8d42a3b7ccb2bee6479306b6ac1a319443702ec
GitHub-Pull-Request: golang/go#51827
Reviewed-on: https://go-review.googlesource.com/c/go/+/394114


Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>