Snapshotted from Go commit 619c7a48a38b28b521591b490fd14ccb7ea5e821
(https://go-review.googlesource.com/c/go/+/229762,
"crypto/x509: add x509omitbundledroots build tag to not embed roots")
With 975c01342a25899962969833d8b2873dc8856a4f
(https://go-review.googlesource.com/c/go/+/220721) removed, because it
depends on other stuff in Go std that doesn't yet exist in a Go
release.
Also, add a subset fork of Go's internal/testenv, for use by x509's tests.
