Move proxy_server_unittest.cc to net/base.

It was previously in a different subdirectory from the file it tests.

Change-Id: Ia5fe22ab35ce774456ce8e19b733d73edbee8e3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333060
Commit-Queue: Eric Roman <eroman@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Auto-Submit: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793765}

net/BUILD.gn

@@ -4153,6 +4153,7 @@ test("net_unittests") {
     "base/prioritized_dispatcher_unittest.cc",
     "base/prioritized_task_runner_unittest.cc",
     "base/priority_queue_unittest.cc",
+    "base/proxy_server_unittest.cc",
     "base/registry_controlled_domains/registry_controlled_domain_unittest.cc",
     "base/scheme_host_port_matcher_rule_unittest.cc",
     "base/scheme_host_port_matcher_unittest.cc",
@@ -4330,7 +4331,6 @@ test("net_unittests") {
     "proxy_resolution/proxy_config_unittest.cc",
     "proxy_resolution/proxy_info_unittest.cc",
     "proxy_resolution/proxy_list_unittest.cc",
-    "proxy_resolution/proxy_server_unittest.cc",
     "quic/bidirectional_stream_quic_impl_unittest.cc",
     "quic/crypto/proof_test_chromium.cc",
     "quic/crypto/proof_verifier_chromium_test.cc",