add PatchPath method

testing/testbase/cleanup.go

Index: testing/testbase/cleanup.go
=== modified file 'testing/testbase/cleanup.go'
--- testing/testbase/cleanup.go	2014-01-20 02:39:11 +0000
+++ testing/testbase/cleanup.go	2014-02-18 17:08:55 +0000
@@ -62,6 +62,13 @@
 	s.AddCleanup(func(*gc.C) { restore() })
 }
 
+// PatchEnvPathPrepend prepends the given path to the environment $PATH and restores the
+// original path on test teardown.
+func (s *CleanupSuite) PatchEnvPathPrepend(dir string) {
+	restore := PatchEnvPathPrepend(dir)
+	s.AddCleanup(func(*gc.C) { restore() })
+}
+
 // PatchValue sets the 'dest' variable the the value passed in. The old value
 // is saved and returned to the original value at test tear down time using a
 // cleanup function. The value must be assignable to the element type of the