lib: consistent `this` in callbacks

This change normalizes a few situations where a callback might be invoked with this set to undefined in one case, but null in a very similar case.

First commit:

test: refactor test-fs-stat

* add `use strict'
* change checks that `this` is mapped to `global` in sloppy mode to
  checks that `this` is `undefined`
* modify arguments to assertions to match docs (actual first, expected
  second)
* add blank line below `common` declaration per test writing guide
* use `assert.ifError()` as appropriate

Second commit:

fs: invoke callbacks with undefined context

Many callbacks appear to be invoked with `this` set to `undefined`
including `fs.stat()`, `fs.lstat()`, and `fs.fstat()`.

However, some such as `fs.open()` and `fs.mkdtemp()` invoke their
callbacks with `this` set to `null`.

Use a consistent way to invoke callbacks to guarantee consistency.

Third commit:

test: check `this` value for `nextTick()`

Depending on how many arguments are provided, `nextTick()` may run its
callback with `this` set to `null` or not. Add assertions for
both cases.

Fourth commit:

process: make `this` value consistent

The value of `this` for callbacks of `nextTick()` can vary depending on
the number of arguments. Make it consistent.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test process lib fs