TT#47373 Fix 0bc0550: add default for variable NGCP_TESTSUITE

Otherwise ngcpcfg fails in trunk with error:
> +03:29:10 (cfg.inc:378): cfg_common_configuration():  ngcpcfg set /etc/ngcp-config/config.yml bootenv.netscript.fallbackfssize=10M
> Save size of 'code' partitions to /etc/ngcp-config/config.yml
> /usr/share/ngcp-ngcpcfg/functions//main: line 68: NGCP_TESTSUITE: unbound variable

Change-Id: I32961cda14550278662acd40ed5863c19412de3c

functions/main

@@ -65,7 +65,7 @@ hook_setup() {
   fi
 
   if ! [ -d "$target_directory" ] ; then
-    if [ "${NGCP_TESTSUITE}" = "true" ]; then
+    if [ "${NGCP_TESTSUITE:-false}" = "true" ]; then
       log_info "Hook target directory $target_directory not a directory. Creating it."
       mkdir -p "$target_directory"
     else