internal/flow/manager: Change test that uses same manager for dialer and acceptor.
internal/flow/manager: Change test that uses same manager for dialer
and acceptor.

When a manager is used for both ends of a conn, one conn gets overwritten
in the cache, causing the test to leak goroutines.
Until we decide upon the right way to deal with this, change the test
so that go-race can become green again.

Change-Id: Ib52b5642951e41922732f4b19a9cafaf16662376

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref