xcm-emulator: add Rococo<>Westend bridge and add tests for assets transfers over the bridge (#2251)

- switch from Rococo<>Wococo to Rococo<>Westend bridge
- add bidirectional simple tests
- remove Wococo chains from xcm-emulator
- added tests for assets transfers over Rococo<>Westend bridge 

fixes https://github.com/paritytech/parity-bridges-common/issues/2405