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.
Fixes: https://github.com/nodejs/node/issues/10742
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test fs