Update the internals of the project to protect against the potential of
polluted properties based on manual evaluation of the source code. Re-
use the `hasOwn` function from `options.js` (moved to `reflection.js`)
to achieve this.

- `executables.js`: Explicitly check that `PATH` (or `Path`) isn't being
  inherited. This is the most obvious place where this is necessary as
  the code already accounts for the potential of `PATH` being undefined.
- `platforms.js`: Safely get `OSTYPE` in case it isn't defined in order
  to avoid wrongly concluding the current system is a Windows system as
  a result of a polluted `OSTYPE` value.
- `win.js`: Safely get `ComSpec` in case it isn't defined (already taken
  into account as well) to avoid using a default shell defined by a
  polluted property.

- Test that when `PATH` and `Path` are missing, no path is given to
  `which`.
- Test that a polluted `PATH` value is not used.
- Test that a polluted `Path` value is not used.
- Simplify existing tests that unnecessarily require `PATH` and `Path`
  to be present on the path.

Test that a polluted `ComSpec` value is not used.

Also, improve upon [1] by
- Not requiring non-empty PATH strings
- Improving test titles for newly added tests
for the `executables.js` test suite.

--
1. e553a73

Otherwise they may fail. No matter how unlikely, we don't want these
tests to be flaky in practice.

I've opted to use `fc` within an AVA `test` over `testProp` because
`fc.pre()` does not seem to work with `testProp` (test fails if check
fails, instead of skipping it) and I think using `fc.pre()` is more
expressive than filtering the arbitrary (though it has the same result
and does work with `testProp`).

- Correct OSTYPE pollution test, arbitrary values were ordered incorrect
- Improve ComSpec pollution test, include when ComSpec is defined
- Cover scenario where a value is both defined and polluted by