test: don't expand the subshell expression prematurely

test: don't expand the subshell expression prematurely

We need to expand the subshell expression during the `bash -c`
invocation, not before, to take the desired effect, as now it expands to:

timeout 30 bash -c 'while [[ 0 -eq 0 ]]; do sleep 1; done'

instead of the expected:

timeout 30 bash -c 'while [[ $(coredumpctl list -q --no-legend 770 | wc -l) -eq 0 ]]; do sleep 1; done'

Follow-up to aadbd81f7f.