HostPortSuite.TestAddressesWithPortAndHostsWithoutPort panic on windows

[Nicholas Skaggs]

As seen in http://reports.vapour.ws/releases/issue/58935076749a5640cbf67e47:

PANIC: <autogenerated>:41: HostPortSuite.SetUpTest

... Panic: Couldn't create temporary directory: mkdir �\check-5765484004404056823: invalid argument (PC=0x45D115)

C:/Go/src/runtime/asm_amd64.s:472
  in call32
C:/Go/src/runtime/panic.go:443
  in gopanic
c:/users/admini~1/appdata/local/temp/tmpclqg_u/gogo/src/github.com/juju/juju/testing/base.go:59
  in JujuOSEnvSuite.SetUpTest
c:/users/admini~1/appdata/local/temp/tmpclqg_u/gogo/src/github.com/juju/juju/testing/base.go:155
  in BaseSuite.SetUpTest
C:/Go/src/runtime/asm_amd64.s:472
  in call32
C:/Go/src/reflect/value.go:435
  in Value.call
C:/Go/src/reflect/value.go:303
  in Value.Call
C:/Go/src/runtime/asm_amd64.s:1998
  in goexit

----------------------------------------------------------------------
PANIC: hostport_test.go:279: HostPortSuite.TestAddressesWithPortAndHostsWithoutPort

... Panic: Fixture has panicked (see related PANIC)

[Anastasia (anastasia-macmood)]

PR against 2.1: https://github.com/juju/juju/pull/6909