Bug 1503613: Make 'mach xpcshell-test' print the command in shell syntax. r=ted

When passed the `--verbose` flag, `mach xpcshell-test` already prints the
current directory, environment variables, and command used for the test.
However, it prints them in Python syntax. This seems like a good thing to do, as
it's a clear indication of what the test is actually doing. But if one wants to
reproduce a problem by running the xpcshell command directly, it's an
error-prone pain to convert from Python lists of strings to the Bourne shell
syntax.

Fortunately, the Python 2.7 `pipes` module has a function, `pipes.quote`, which
produces properly quoted Unix shell commands, ready to be copied and pasted into
one's terminal.

Unfortunately, the xpcshell tests still clean up the $obj/temp/xpc-plugins-FOO
directory, so this still isn't quite ready to go.

Differential Revision: https://phabricator.services.mozilla.com/D10392

--HG--
extra : moz-landing-system : lando

testing/xpcshell/runxpcshelltests.py

@@ -10,6 +10,7 @@ import copy
 import json
 import mozdebug
 import os
+import pipes
 import random
 import re
 import shutil
@@ -285,6 +286,9 @@ class XPCShellTestThread(Thread):
         changedEnv = (set("%s=%s" % i for i in self.env.iteritems())
                       - set("%s=%s" % i for i in os.environ.iteritems()))
         self.log.info("%s | environment: %s" % (name, list(changedEnv)))
+        shell_command_tokens = [pipes.quote(tok) for tok in list(changedEnv) + completeCmd]
+        self.log.info("%s | as shell command: (cd %s; %s)" %
+                      (name, pipes.quote(testdir), ' '.join(shell_command_tokens)))
 
     def killTimeout(self, proc):
         if proc is not None and hasattr(proc, "pid"):