(core) Add test_replay for easily replaying data sent to the sandbox purely within python

Summary:
Run JS with a value for SANDBOX_BUFFERS_DIR, then run test_replay in python with the same value to replay just the python code.

See test_replay.py for more info.

Test Plan:
Record some data, e.g. `SANDBOX_BUFFERS_DIR=manual npm start` or `SANDBOX_BUFFERS_DIR=server ./test/testrun.sh server`.

Then run `SANDBOX_BUFFERS_DIR=server python -m unittest test_replay` from within `core/sandbox/grist` to replay the input from the JS.

Sample of the output will look like this:

```
Checking /tmp/sandbox_buffers/server/2021-06-16T15:13:59.958Z
True
Checking /tmp/sandbox_buffers/server/2021-06-16T15:16:37.170Z
True
Checking /tmp/sandbox_buffers/server/2021-06-16T15:14:22.378Z
True
```

Reviewers: paulfitz, dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2866

sandbox/grist/sandbox.py

@@ -11,7 +11,6 @@ Usage:
 
 import os
 import marshal
-import signal
 import sys
 import traceback
 
@@ -36,10 +35,16 @@ class Sandbox(object):
   DATA = True
   EXC = False
 
-  def __init__(self):
+  def __init__(self, external_input, external_output):
     self._functions = {}
-    self._external_input = os.fdopen(3, "rb", 64*1024)
-    self._external_output = os.fdopen(4, "wb", 64*1024)
+    self._external_input = external_input
+    self._external_output = external_output
+
+  @classmethod
+  def connected_to_js_pipes(cls):
+    external_input = os.fdopen(3, "rb", 64 * 1024)
+    external_output = os.fdopen(4, "wb", 64 * 1024)
+    return cls(external_input, external_output)
 
   def _send_to_js(self, msgCode, msgBody):
     # (Note that marshal version 2 is the default; we specify it explicitly for clarity. The
@@ -87,14 +92,19 @@ class Sandbox(object):
     if break_on_response:
       raise Exception("Sandbox disconnected unexpectedly")
 
+default_sandbox = None
 
-sandbox = Sandbox()
+def get_default_sandbox():
+  global default_sandbox
+  if default_sandbox is None:
+    default_sandbox = Sandbox.connected_to_js_pipes()
+  return default_sandbox
 
 def call_external(name, *args):
-  return sandbox.call_external(name, *args)
+  return get_default_sandbox().call_external(name, *args)
 
 def register(func_name, func):
-  sandbox.register(func_name, func)
+  get_default_sandbox().register(func_name, func)
 
 def run():
-  sandbox.run()
+  get_default_sandbox().run()