v4.x backport - test: convert var->const/let in tests

[gibfahn]

I'll fix up the commit messages once the tests are passing.

Backport of #10685, I couldn't cherry-pick as it brought in hundreds of unrelated changes, so I redid the eslint --fix and manual fixup for v4.x.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

[gibfahn]

I'm getting an error for tests that have strict mode disabled, e.g. test-fs-stat#L40. I resolved it by disabling the no-var lint rule for those lines. If anyone objects please let me know.

=== release test-fs-stat ===                                                   
Path: parallel/test-fs-stat
/Users/gib/wrk/com/node/test/parallel/test-fs-stat.js:40
  let stats;
  ^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:373:25)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:140:18)
    at node.js:1043:3
Command: out/Release/node /Users/gib/wrk/com/node/test/parallel/test-fs-stat.js

Files failing:

• test/parallel/test-fs-stat.js
• test/parallel/test-module-nodemodulepaths.js
• test/parallel/test-net-binary.js
• test/parallel/test-repl.js
• test/parallel/test-util-inspect.js

[gibfahn]

CI: https://ci.nodejs.org/job/node-test-commit/8345/

I'd appreciate people taking a look at the last three commits.

[MylesBorins]

@gibfahn since v4.8.1 is likely going to be the last Active LTS release of v4.x and the first r.c. has already gone out it might make sense to skip this.

[gibfahn]

@MylesBorins obviously I'd like you to merge it if possible (having just finished wrestling it into shape), but it's up to you. The benefit to having this is that virtually nothing will backport cleanly to v4.x without it (same is true to a lesser extent of #10698 as well). Should also be low-risk as it's just testcase changes.

However I guess if we're really not expecting almost anything to be added to v4.x in the future I guess it might not be worth it, ¯\_(ツ)_/¯

EDIT: Whichever it is, let me know whether you want #10698 backported to v4.x, that one should be easier.

[gibfahn]

I've rebased and rerun CI anyway, so @MylesBorins feel free to whatever you think is best!

CI 2: https://ci.nodejs.org/job/node-test-commit/8360/

The branch from before I squashed is var2const-v4.x-presquash, the squash! manual commits are the ones that might be worth reviewing.

[MylesBorins]

I'm going to go ahead and close this for v4, will land the v6 version after this release