[FrameworkBundle] added a test router for the built-in web server

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9995
| License       | MIT
| Doc PR        | n/a

#9995 describes a very valid use case.

Not sure if this should be merged into 2.7 only... now that I think about it, probably :)