test: move resource intensive test to sequential

test-fs-readfile-tostring-fail is resource-intensive and is flaky in
CI. Move to sequential tests so it is not competing for resources with
other tests.

PR-URL: nodejs#10744
Fixes: nodejs#10742
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Brian White <[email protected]>
Reviewed-By: Santiago Gimeno <[email protected]>

Author: Trott