tests: let "run-nm-test.sh" fail with exit code 1 on failure

`git bisect run` is peculiar about the exit code:

  error: bisect run failed: exit code 134 from '...' is < 0 or >= 128

If we just "exec" the test, it usually will fail on an assert. That results
in SIGABRT or exit code 134. So out of the box that is annoying with
git-bisect.

Work around that and let the test wrapper always coerce any test failure
to exit code 1.

tools/run-nm-test.sh

@@ -300,9 +300,10 @@ fi
 
 if ! _is_true "$NMTST_USE_VALGRIND" 0; then
     export NM_TEST_UNDER_VALGRIND=0
-    exec "${NMTST_DBUS_RUN_SESSION[@]}" \
-    "$TEST" "${TEST_ARGV[@]}"
-    die "exec \"$TEST\" failed"
+    "${NMTST_DBUS_RUN_SESSION[@]}" "$TEST" "${TEST_ARGV[@]}"
+    r=$?
+    [ $r == 0 ] || die "exec \"$TEST\" failed with exit code $r"
+    exit 0
 fi
 
 if [[ -z "${NMTST_VALGRIND}" ]]; then