Start work on prewarming

The prewarm process and its controller are implemented with some basic
tests.

kitty_tests/prewarm.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python
+# License: GPLv3 Copyright: 2022, Kovid Goyal <kovid at kovidgoyal.net>
+
+
+import json
+import os
+import tempfile
+
+from kitty.constants import kitty_exe
+
+from . import BaseTest
+
+
+class Prewarm(BaseTest):
+
+    maxDiff = None
+
+    def test_prewarming(self):
+        from kittens.prewarm.main import PrewarmProcess
+
+        p = PrewarmProcess(create_file_to_read_from_worker=True)
+        cwd = tempfile.gettempdir()
+        env = {'TEST_ENV_PASS': 'xyz'}
+        cols = 117
+        stdin_data = 'from_stdin'
+        pty = self.create_pty(cols=cols)
+        ttyname = os.ttyname(pty.slave_fd)
+        child = p(pty.slave_fd, [kitty_exe(), '+runpy', """import os, json; from kitty.utils import *; print(json.dumps({
+        'cterm': os.ctermid(),
+        'ttyname': os.ttyname(sys.stdout.fileno()),
+        'cols': read_screen_size().cols,
+        'cwd': os.getcwd(),
+        'env': os.environ.get('TEST_ENV_PASS'),
+        'pid': os.getpid(),
+        'stdin': sys.stdin.read(),
+
+        'done': 'hello',
+        }, indent=2))"""], cwd=cwd, env=env, stdin_data=stdin_data)
+        self.assertFalse(pty.screen_contents().strip())
+        p.mark_child_as_ready(child.child_id)
+        pty.wait_till(lambda: 'hello' in pty.screen_contents())
+        data = json.loads(pty.screen_contents())
+        self.ae(data['cols'], cols)
+        self.assertTrue(data['cterm'])
+        self.ae(data['ttyname'], ttyname)
+        self.ae(data['cwd'], cwd)
+        self.ae(data['env'], env['TEST_ENV_PASS'])
+        self.ae(int(p.from_worker.readline()), data['pid'])