test: reduce flakiness of `test-fs-read-position-validation.mjs`
The said test failed randomly several times with EBADF
error code: 1(https://ci.nodejs.org/job/node-test-commit-linux-containered/31769/nodes=ubuntu1804_sharedlibs_zlib_x64/testReport/junit/(root)/test/parallel_test_fs_read_position_validation/), 2(https://ci.nodejs.org/job/node-test-commit-linux/45657/nodes=alpine-last-latest-x64/testReport/junit/(root)/test/parallel_test_fs_read_position_validation/), 3(https://ci.nodejs.org/job/node-test-commit-linux-containered/31735/nodes=ubuntu1804_sharedlibs_openssl111_x64/testReport/junit/(root)/test/parallel_test_fs_read_position_validation/)
This PR should make the test more reliable by promisifying Callback API parts and calling them in sequence. Additionally, it separates Callback API and Synchronous API parts into two tests, in anticipation of validation in Promises API.