Add a testbase package with no juju-core deps.

juju/osenv/package_test.go

 // Copyright 2013 Canonical Ltd.
 // Licensed under the AGPLv3, see LICENCE file for details.
 
 package osenv_test
 
 import (
         "testing"
 
         gc "launchpad.net/gocheck"
 
-        coretesting "launchpad.net/juju-core/testing"
+        "launchpad.net/juju-core/testing/testbase"
 )
 
 func Test(t *testing.T) {
         gc.TestingT(t)
 }
 
 type importSuite struct{}
 
 var _ = gc.Suite(&importSuite{})
 
 func (*importSuite) TestDependencies(c *gc.C) {
         // This test is to ensure we don't bring in dependencies at all.
-        c.Assert(coretesting.FindJujuCoreImports(c, "launchpad.net/juju-core/juju/osenv"),
+        c.Assert(testbase.FindJujuCoreImports(c, "launchpad.net/juju-core/juju/osenv"),
                 gc.HasLen, 0)
 }