test: refactor header inclusion to speed up building

[foreseeable]

Signed-off-by: Muge Chen mugechen@google.com

Refactor mock class include directive after dividing test/server/mocks.h (#11649)

Building time comparison for affected tests:

test_name	before	after
//test/common/upstream:cluster_factory_impl_test	97.051s	80.938s
//test/common/upstream:eds_speed_test	102.03s	75.825s
//test/common/upstream:eds_test	101.319s	77.988s
//test/common/upstream:hds_test	105.666s	74.24s
//test/common/upstream:logical_dns_cluster_test	97.184s	78.586s
//test/common/upstream:original_dst_cluster_test	116.306s	74.845s
//test/common/upstream:transport_socket_matcher_test	99.237s	67.548s
//test/common/upstream:upstream_impl_test	101.438s	82.394s

Commit Message: refactor header inclusion to speed up building
Additional Description:
Risk Level: low
Testing: exsiting tests
Docs Changes: N/A
Release Notes: no
Related Issues: #10917

\cc @ahedberg

[ggreenway]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ggreenway]

Looks good. Thanks for working on this.

The CI failure looks like a flake; it compiled, so I don't think this PR could be the cause.