Move the SocketStream class to test target

It's used only by the rtc_base/ssladapter_unittest.cc.

Bug: webrtc:9838
Change-Id: Ic89549e1a32003ec0771ca6ba619fecfcf95b493
Reviewed-on: https://webrtc-review.googlesource.com/c/106924
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25267}

rtc_base/BUILD.gn

@@ -801,8 +801,6 @@ rtc_static_library("rtc_base") {
     "socketaddresspair.h",
     "socketfactory.h",
     "socketserver.h",
-    "socketstream.cc",
-    "socketstream.h",
     "ssladapter.cc",
     "ssladapter.h",
     "sslcertificate.cc",
@@ -966,6 +964,8 @@ rtc_source_set("rtc_base_tests_utils") {
     "proxyserver.h",
     "sigslottester.h",
     "sigslottester.h.pump",
+    "socketstream.cc",
+    "socketstream.h",
     "testbase64.h",
     "testcertificateverifier.h",
     "testclient.cc",