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/internal/platform/platform_test.go

@@ -0,0 +1,68 @@
+package platform
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestGood(t *testing.T) {
+	var expected bool
+	switch Name() {
+	case "android", "ios", "linux", "macos", "windows":
+		expected = true
+	}
+	if !expected {
+		t.Fatal("unexpected platform name")
+	}
+}
+
+func TestPuregoname(t *testing.T) {
+	var runtimevariables = []struct {
+		expected string
+		goarch   string
+		goos     string
+	}{{
+		expected: "android",
+		goarch:   "*",
+		goos:     "android",
+	}, {
+		expected: "ios",
+		goarch:   "arm64",
+		goos:     "darwin",
+	}, {
+		expected: "ios",
+		goarch:   "arm",
+		goos:     "darwin",
+	}, {
+		expected: "linux",
+		goarch:   "*",
+		goos:     "linux",
+	}, {
+		expected: "macos",
+		goarch:   "amd64",
+		goos:     "darwin",
+	}, {
+		expected: "macos",
+		goarch:   "386",
+		goos:     "darwin",
+	}, {
+		expected: "unknown",
+		goarch:   "*",
+		goos:     "solaris",
+	}, {
+		expected: "unknown",
+		goarch:   "mips",
+		goos:     "darwin",
+	}, {
+		expected: "windows",
+		goarch:   "*",
+		goos:     "windows",
+	}}
+	for _, v := range runtimevariables {
+		t.Run(fmt.Sprintf("with %s/%s", v.goos, v.goarch), func(t *testing.T) {
+			if puregoname(v.goos, v.goarch) != v.expected {
+				t.Fatal("unexpected results")
+			}
+		})
+	}
+}