test: cleanup roundrobin tests to use stubserver

[easwars]

Summary of changes:

• Delete the existing tests in balancer/roundrobin/roundrobin_test.go and move them to test/roundrobin_test.go.
  • We are not testing anything in these tests which are internal to the roundrobin package. So, making these tests e2e sounds like a better option.
• Get rid of the custom implementation of TestService, and use the stubserver package instead.
• Add some test helpers to make the tests more readable.

Old tests were 591 lines long, the new one is 411 lines long.

#stubserver-everywhere

RELEASE NOTES: none