testenv: Fix missing import module util

util module is used in prompt() method (debug suite, interactive test).

Change-Id: I9bb759d053a99108f121f1c4f7994726b6075fee
diff --git a/src/osmo_gsm_tester/testenv.py b/src/osmo_gsm_tester/testenv.py
index 1790059..a74891d 100644
--- a/src/osmo_gsm_tester/testenv.py
+++ b/src/osmo_gsm_tester/testenv.py
@@ -23,6 +23,7 @@
 
 import sys
 
+from .core import util
 from .core import process
 from .core import template
 from .core import log as log_module