This patch:
- updates documentation to lead users from `TestConfig.snapshotDir` and
`testInfo.snapshotSuffix` to `TestConfig.snapshotPathTemplate` as a
better and more flexible alternative.
- drops `{snapshotSuffix}` from documentation
- stops using `snapshotSuffix = ''` in our own tests and switches us
to the `snapshotPathTemplate`.
- adds `{testName}` token.
