runtime/secret: reorganize tests to fix -buildmode=shared

runtime/secret: reorganize tests to fix -buildmode=shared

The testing assembly methods had a linkname that was implicitly
satisfied during the regular build but not there during the shared
build. Fix by moving the testing routine into the package itself.

For good measure, section off the assembly files from the non-experiment
build. Should prevent further build failures as we work on this.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-arm64-longtest
Change-Id: I2b45668e44641ae7880ff14f6402d982c7eaedd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/724001
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>