The new methods [T.ArtifactDir], [B.ArtifactDir], and [F.ArtifactDir]
return a directory in which to write test output files (artifacts).

When the `-artifacts` flag is provided to `go test`,
this directory will be located under the output directory
(specified with `-outputdir`, or the current directory by default).
Otherwise, artifacts are stored in a temporary directory
which is removed after the test completes.

The first call to `ArtifactDir` when `-artifacts` is provided
writes the location of the directory to the test log.

For example, in a test named `TestArtifacts`,
`t.ArtifactDir()` emits:

```
=== ARTIFACTS Test /path/to/artifact/dir
```