run-nm-test: fix using exec instead of running and exiting

Otherwise, the script tries to run

  dbus-run-session -- exec ...

which fails (because `exec` is a shell command, not a program).
After the failure, the code falls through to run the test under
valgrind.

Fixes: 6a58c55ca4 ('run-nm-test: Just use exec instead of running and exiting')

tools/run-nm-test.sh

@@ -230,8 +230,8 @@ fi
 
 if ! _is_true "$NMTST_USE_VALGRIND" 0; then
     export NM_TEST_UNDER_VALGRIND=0
-    "${NMTST_DBUS_RUN_SESSION[@]}" \
-    exec "$TEST" "$@"
+    exec "${NMTST_DBUS_RUN_SESSION[@]}" \
+    "$TEST" "$@"
 fi
 
 if [[ -z "${NMTST_VALGRIND}" ]]; then