`options.cwd` should use the canonical pathname

`TextExclude.shouldInstrument(filename)` returns `false` when `options.relativePath` is `true` and `filename` is not inside `options.cwd`.

However:
- when `options.cwd` is a symlink and is passed with its symlink unresolved
- but `filename` resolves the symlink (through `fs.realpath()` for example)
- then `shouldInstrument()` will return `false` even though it should return `true`.

This is likely to happen since:
- the filename passed by `nyc` always has symlinks resolved because that's how `module._compile()` works
- however `nyc` user might pass a `options.cwd` that has an unresolved symlink.

Real-life example: running the following on Travis on Mac OS:

```js
const os = require('os');
const execa = require('execa');

const cwd = os.tmpdir();
await execa.shell(`nyc --cwd ${cwd}`)
```

Travis Mac OS environment:
- returns `/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T` with `os.tmpdir()`
- but this directory is a symlink for `/private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T`
- as a consequence all `filename` passed to `testExclude.shouldInstrument()` will start with `/private/var/...`, while `options.cwd()` will start with `/var/...`, even though it's the same folder.

One possible solution would be to run `fs.realpath()` on both `filename` and `options.cwd()` before comparing them.

Start by tracing TextExclude.shouldInstrument(filename), focusing on the comparison between options.cwd and filename when options.relativePath is true. Check how fs.realpath() can canonicalize both paths, then verify that equivalent symlinked paths are treated as inside the configured cwd; the payload does not name a test file to run.

Written by the indexing model from the issue text.