RebootSuite test failures on windows

[Martin Packman]

TestRebootNoContainers and TestShutdownNoContainers started failing in CI on the windows run unit tests job.

<http://reports.vapour.ws/releases/issue/55e6f78d749a56422bf88973>

FAIL: reboot_windows_test.go:35: RebootSuite.TestRebootNoContainers
...
reboot_windows_test.go:42:
    testing.AssertEchoArgs(c, rebootBin, expectedRebootParams...)
c:/users/admini~1/appdata/local/temp/tmphsfuse/gogo/src/github.com/juju/testing/cmd.go:194:
    c.Assert(actual, gc.Equals, expected)
... obtained string = "shutdown.exe \"-f\" \"-r\" \"-t\" \"15\""
... expected string = "shutdown.exe '-f' '-r' '-t' '15'"
...
FAIL: reboot_windows_test.go:45: RebootSuite.TestShutdownNoContainers
...
reboot_windows_test.go:52:
    testing.AssertEchoArgs(c, rebootBin, expectedShutdownParams...)
c:/users/admini~1/appdata/local/temp/tmphsfuse/gogo/src/github.com/juju/testing/cmd.go:194:
    c.Assert(actual, gc.Equals, expected)
... obtained string = "shutdown.exe \"-f\" \"-s\" \"-t\" \"15\""
... expected string = "shutdown.exe '-f' '-s' '-t' '15'"
...
FAIL github.com/juju/juju/cmd/jujud/reboot 4.279s

Related to the dependecy update to juju/testing from pr #3170:

<https://github.com/juju/juju/commit/cbd011823ef2b9645fde7824054109d3c5c2ae84>

Which included this quoting change:

<https://github.com/juju/testing/pull/73>

Juju tests just need updating.