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: https://github.com/nodejs/node/pull/10744
Fixes: https://github.com/nodejs/node/issues/10742


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>