[Framework] Fixed a Stopwatch namespace

Also, added a test.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes (they're actually failing on PHP 5.3 but not because of this PR)
| Fixed tickets | none
| License       | MIT
| Doc PR        | none