test: add check to test-fs-readfile-tostring-fail
Check that all of the bytes were written to the temporary file before reading it to catch the case where there is insufficient disk space.
Refs: https://github.com/nodejs/node/issues/43833
It feels like this extra check shouldn't be necessary but for some reason I can't get this test to emit any errors on the write stream cc @nodejs/streams @nodejs/fs.