internal/health: use http.Get instead of s.Client.Get

httptest.Server.Client was added in Go 1.9, and we want to support Go
1.8+ for App Engine.

Change-Id: I1eb539a69c2dadc85505b181eeeb6e20dd333df7
Reviewed-on: https://go-review.googlesource.com/78539
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>