let tests override KEY_TIMER/VERIFY_TIMER to tolerate slow test hosts

I've seen intermittent failures in
test_cli.PregeneratedCode.test_text_subprocess where the host was slow (or
overloaded) enough that the "Waiting for sender.." pacifier message was
displayed, which flunks the test because we're looking for a specific output
string. We patch this 1-second timer in the non-subprocess tests, but you
can't patch across a process boundary.

This patch adds an undocumented environment variable that lets you override
the timer values. The test then sets it to something large.

For future consideration: another approach would be to change the test to
tolerate the extra message. This would be trickier to validate, though.

src/wormhole/cli/cmd_receive.py

@@ -14,8 +14,8 @@ from .welcome import handle_welcome
 
 APPID = u"lothar.com/wormhole/text-or-file-xfer"
 
-KEY_TIMER = 1.0
-VERIFY_TIMER = 1.0
+KEY_TIMER = float(os.environ.get("_MAGIC_WORMHOLE_TEST_KEY_TIMER", 1.0))
+VERIFY_TIMER = float(os.environ.get("_MAGIC_WORMHOLE_TEST_VERIFY_TIMER", 1.0))
 
 class RespondError(Exception):
     def __init__(self, response):

src/wormhole/cli/cmd_send.py

@@ -13,7 +13,7 @@ from ..util import dict_to_bytes, bytes_to_dict, bytes_to_hexstr
 from .welcome import handle_welcome
 
 APPID = u"lothar.com/wormhole/text-or-file-xfer"
-VERIFY_TIMER = 1
+VERIFY_TIMER = float(os.environ.get("_MAGIC_WORMHOLE_TEST_VERIFY_TIMER", 1.0))
 
 def send(args, reactor=reactor):
     """I implement 'wormhole send'. I return a Deferred that fires with None

src/wormhole/test/test_cli.py

@@ -468,6 +468,15 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
             elif send_cfg.what:
                 content_args = [send_cfg.what]
 
+            # raise the rx KEY_TIMER to some large number here, to avoid
+            # spurious test failures on hosts that are slow enough to trigger
+            # the "Waiting for sender..." pacifier message. We can do in
+            # not-as_subprocess, because we can directly patch the value before
+            # running the receiver. But we can't patch across the subprocess
+            # boundary, so we use an environment variable.
+            env = self._env.copy()
+            env["_MAGIC_WORMHOLE_TEST_KEY_TIMER"] = "999999"
+            env["_MAGIC_WORMHOLE_TEST_VERIFY_TIMER"] = "999999"
             send_args = [
                     '--relay-url', self.relayurl,
                     '--transit-helper', '',
@@ -479,7 +488,7 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
             send_d = getProcessOutputAndValue(
                 wormhole_bin, send_args,
                 path=send_dir,
-                env=self._env,
+                env=env,
             )
             recv_args = [
                 '--relay-url', self.relayurl,
@@ -495,7 +504,7 @@ class PregeneratedCode(ServerBase, ScriptsBase, unittest.TestCase):
             receive_d = getProcessOutputAndValue(
                 wormhole_bin, recv_args,
                 path=receive_dir,
-                env=self._env,
+                env=env,
             )
 
             (send_res, receive_res) = yield gatherResults([send_d, receive_d],