crypto/tls: fix test caching for bogo

crypto/tls: fix test caching for bogo

Use a stat to tell the test caching infrastructure that we care about
the bogo config file.

Change-Id: Iae2336bfc45cd6a9e73cb83a3b68ade8b4f23927
Reviewed-on: https://go-review.googlesource.com/c/go/+/589155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>