test: refactor test-fs-read-stream-inherit

Refactor to take advantage of block scoping to isolate tests. Checks in
exit handlers now reside with the relevant test block. Where test cases
start and end is more clear.

Also: Some use of `common.mustCall()` and improved wrapping/indentation.

PR-URL: https://github.com/nodejs/node/pull/10246


Reviewed-By: Michaël Zasso <targos@protonmail.com>