Now that Pebble has a `pebble-challtestsrv` we can remove the `challtestrv`
package and associated command from Boulder. I switched CI to use
`pebble-challtestsrv`. Notably this means that we have to add our expected mock
data using the HTTP management interface. The Boulder-tools images are
regenerated to include the `pebble-challtestsrv` command.
Using this approach also allows separating the TLS-ALPN-01 and HTTPS HTTP-01
challenges by binding each challenge type in the `pebble-challtestsrv` to
different interfaces both using the same VA
HTTPS port. Mock DNS directs the VA to the correct interface.
The load-generator command that was previously using the `challtestsrv` package
from Boulder is updated to use a vendored copy of the new
`github.org/letsencrypt/challtestsrv` package.
Vendored dependencies change in two ways:
1) Gomock is updated to the latest release (matching what the Bouldertools image
provides)
2) A couple of new subpackages in `golang.org/x/net/` are added by way of
transitive dependency through the challtestsrv package.
Unit tests are confirmed to pass for `gomock`:
```
~/go/src/github.com/golang/mock/gomock$ git log --pretty=format:'%h' -n 1
51421b9
~/go/src/github.com/golang/mock/gomock$ go test ./...
ok github.com/golang/mock/gomock 0.002s
? github.com/golang/mock/gomock/internal/mock_matcher [no test files]
```
For `/x/net` all tests pass except two `/x/net/icmp` `TestDiag.go` test cases
that we have agreed are OK to ignore.
Resolves https://github.com/letsencrypt/boulder/issues/3962 and
https://github.com/letsencrypt/boulder/issues/3951
Daniel McCarney