Refactor service tests in `sc-network` (#12517)

* Refactor service tests in `sc-network`

Create a separate directory for the tests and move common network
instantion related code to `mod.rs` from where it can be used by both
service and chainsync tests.

Use the builder pattern when creating the `TestNetwork` object to reduce
code duplication between the test files.

* Update client/network/src/service/tests/mod.rs

Co-authored-by: Dmitrii Markin <[email protected]>

Co-authored-by: Dmitrii Markin <[email protected]>
Co-authored-by: parity-processbot <>