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/QA/common.py

@@ -0,0 +1,72 @@
+""" ./QA/common.py - common code for QA """
+
+import contextlib
+import json
+import os
+import shlex
+import shutil
+import socket
+import subprocess
+import sys
+import time
+import urllib.parse
+
+
+def execute(args):
+    """ Execute a specified command """
+    subprocess.run(args)
+
+
+def execute_jafar_and_miniooni(ooni_exe, outfile, experiment, tag, args):
+    """ Executes jafar and miniooni. Returns the test keys. """
+    tmpoutfile = "/tmp/{}".format(outfile)
+    with contextlib.suppress(FileNotFoundError):
+        os.remove(tmpoutfile)  # just in case
+    execute(
+        [
+            "./jafar",
+            "-main-command",
+            "./QA/minioonilike.py {} -n -o '{}' --home /tmp {}".format(
+                ooni_exe, tmpoutfile, experiment
+            ),
+            "-main-user",
+            "nobody",  # should be present on Unix
+            "-tag",
+            tag,
+        ]
+        + args
+    )
+    shutil.copy(tmpoutfile, outfile)
+    result = read_result(outfile)
+    assert isinstance(result, dict)
+    assert isinstance(result["test_keys"], dict)
+    return result["test_keys"]
+
+
+def read_result(outfile):
+    """ Reads the result of an experiment """
+    return json.load(open(outfile, "rb"))
+
+
+def test_keys(result):
+    """ Returns just the test keys of a specific result """
+    return result["test_keys"]
+
+
+def check_maybe_binary_value(value):
+    """ Make sure a maybe binary value is correct """
+    assert isinstance(value, str) or (
+        isinstance(value, dict)
+        and value["format"] == "base64"
+        and isinstance(value["data"], str)
+    )
+
+
+def with_free_port(func):
+    """ This function executes |func| passing it a port number on localhost
+        which is bound but not listening for new connections """
+    # See <https://stackoverflow.com/a/45690594>
+    with contextlib.closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as sock:
+        sock.bind(("127.0.0.1", 0))
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        func(sock.getsockname()[1])