shellcheck: test_switch_id.sh issues

In test_switch_id.sh line 3:
if test "`id -u`" != '0'; then
         ^-- SC2006: Use $(..) instead of legacy `..`.


In test_switch_id.sh line 8:
if test "`id -u nobody`" = ""; then
         ^-- SC2006: Use $(..) instead of legacy `..`.