Reland "[fuchsia] Add ssh debug messages to the emulator output log file."

This is a reland of 844410d2

Fixed the bug in the timeout case logic.  Tested it by reducing the
timeout value so that the timeout logic was exercised.

Original change's description:
> [fuchsia] Add ssh debug messages to the emulator output log file.
>
> This will give more information on startup connectivity issues.
>
> Bug: 1063106
> Change-Id: I90590eaeee7aaac19d8e277b42f42ce5cd6b4f85
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212987
> Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
> Commit-Queue: Stephen Roe <steveroe@google.com>
> Cr-Commit-Position: refs/heads/master@{#775358}

Bug: 1063106
Change-Id: I8a28b9d41d5dce9bd444a04f623f6b260355c6f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235990
Commit-Queue: Stephen Roe <steveroe@google.com>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776267}

build/fuchsia/emu_target.py

@@ -74,7 +74,7 @@ class EmuTarget(target.Target):
                                          env=emu_env)
 
     try:
-      self._WaitUntilReady();
+      self._WaitUntilReady(ssh_diagnostic_log_file=stdout)
     except target.FuchsiaTargetException:
       if temporary_system_log_file:
         logging.info('Kernel logs:\n' +

build/fuchsia/target.py

@@ -211,20 +211,25 @@ class Target(object):
   def _AssertIsStarted(self):
     assert self.IsStarted()
 
-  def _WaitUntilReady(self):
+  def _WaitUntilReady(self, ssh_diagnostic_log_file=open(os.devnull, 'w')):
     logging.info('Connecting to Fuchsia using SSH.')
 
     host, port = self._GetEndpoint()
     end_time = time.time() + _ATTACH_RETRY_SECONDS
     while time.time() < end_time:
       runner = remote_cmd.CommandRunner(self._GetSshConfigPath(), host, port)
-      if runner.RunCommand(['true'], True) == 0:
+      ssh_proc = runner.RunCommandPiped(['true'],
+                                        ssh_args=['-v'],
+                                        stdout=ssh_diagnostic_log_file,
+                                        stderr=subprocess.STDOUT)
+      if ssh_proc.wait() == 0:
         logging.info('Connected!')
         self._started = True
         return True
       time.sleep(_ATTACH_RETRY_INTERVAL)
 
     logging.error('Timeout limit reached.')
+    ssh_diagnostic_log_file.flush()
 
     raise FuchsiaTargetException('Couldn\'t connect using SSH.')