chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/probeservices/testorchestra/testorchestra.go

@@ -0,0 +1,19 @@
+// Package testorchestra helps with testing the OONI orchestra API.
+package testorchestra
+
+import "github.com/ooni/probe-cli/v3/internal/engine/probeservices"
+
+// MetadataFixture returns a valid metadata struct. This is mostly
+// useful for testing. (We should see if we can make this private.)
+func MetadataFixture() probeservices.Metadata {
+	return probeservices.Metadata{
+		Platform:        "linux",
+		ProbeASN:        "AS15169",
+		ProbeCC:         "US",
+		SoftwareName:    "miniooni",
+		SoftwareVersion: "0.1.0-dev",
+		SupportedTests: []string{
+			"web_connectivity",
+		},
+	}
+}