Comment 2 for bug 1301353

The fix might be as simple as:
=== modified file 'testing/mgo.go'
--- testing/mgo.go 2014-04-17 00:19:01 +0000
+++ testing/mgo.go 2014-05-08 15:47:55 +0000
@@ -142,7 +142,12 @@
  if inst.Params != nil {
   mgoargs = append(mgoargs, inst.Params...)
  }
- server := exec.Command("mongod", mgoargs...)
+ path, err := exec.LookPath("mongod")
+ if err != nil {
+ // No mongod on path
+ path, err = exec.LookPath("/usr/lib/juju/bin/mongod")
+ }
+ server := exec.Command(path, mgoargs...)
  out, err := server.StdoutPipe()
  if err != nil {
   return err

But I haven't tested it.