Reland "Move relay server code to a test-only target p2p_server_utils."

This is a reland of e284c521f76d810e9c68a238e4821e8f0f99a2cd

Original change's description:
> Move relay server code to a test-only target p2p_server_utils.
> 
> Bug: webrtc:9798
> Change-Id: I5926cbb11922c7bd1adfa2099431dc461cc63f20
> Reviewed-on: https://webrtc-review.googlesource.com/c/107361
> Reviewed-by: Per Kjellander <perkj@webrtc.org>
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#25347}

Bug: webrtc:9798
Change-Id: I82c6c7d9524217237ad83839cc0fe6f2c184b0e3
Reviewed-on: https://webrtc-review.googlesource.com/c/108300
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26024}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index ce390bc..a2aa5ff 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -109,15 +109,6 @@
     if (is_nacl) {
       deps += [ "//native_client_sdk/src/libraries/nacl_io" ]
     }
-  } else {
-    sources += [
-      "base/relayserver.cc",
-      "base/relayserver.h",
-      "base/stunserver.cc",
-      "base/stunserver.h",
-      "base/turnserver.cc",
-      "base/turnserver.h",
-    ]
   }
 }
 
@@ -139,6 +130,7 @@
       "base/testturnserver.h",
     ]
     deps = [
+      ":p2p_server_utils",
       ":rtc_p2p",
       "../api:libjingle_peerconnection_api",
       "../api:ortc_api",
@@ -181,6 +173,7 @@
       "client/basicportallocator_unittest.cc",
     ]
     deps = [
+      ":p2p_server_utils",
       ":p2p_test_utils",
       ":rtc_p2p",
       "../api:libjingle_peerconnection_api",
@@ -207,6 +200,25 @@
   }
 }
 
+rtc_source_set("p2p_server_utils") {
+  testonly = true
+  sources = [
+    "base/relayserver.cc",
+    "base/relayserver.h",
+    "base/stunserver.cc",
+    "base/stunserver.h",
+    "base/turnserver.cc",
+    "base/turnserver.h",
+  ]
+  deps = [
+    ":rtc_p2p",
+    "../rtc_base:checks",
+    "../rtc_base:rtc_base",
+    "../rtc_base/third_party/sigslot",
+    "//third_party/abseil-cpp/absl/memory",
+  ]
+}
+
 rtc_static_library("libstunprober") {
   visibility = [ "*" ]
   sources = [