Respect --no-clone flag in run_tests.sh

The nailgun repository was being cloned for each test run
regardless of the value of this flag. Rather than testing
this flag for being an empty string compare the value.

Change-Id: I41f904f1d550b6f39fa339c1913407ee0133dcaf

run_tests.sh

@@ -301,11 +301,15 @@ EOL
 obtain_nailgun() {
     echo "Obtaining Nailgun with the revision $fuel_commit"
 
-    if [[ -n $do_clone ]]; then
+    if [[ $do_clone -ne 0 ]]; then
         git clone $FUEL_WEB_REPO $FUEL_WEB_ROOT || \
             { echo "Failed to clone Nailgun"; return 1; }
     fi
 
+    if [[ ! -d "$NAILGUN_ROOT" ]]; then
+        echo "Error: Nailgun directory $NAILGUN_ROOT not found."
+        exit 1
+    fi
     pushd $NAILGUN_ROOT > /dev/null
 
     if [[ -n $fetch_repo ]]; then