Rename confusing socket test filenames

The gio/tests/socket-client.c doesn't use GSocketClient, which makes the
filename confusing. What the file actually tests is the GSocket. Rename
it to socket-testclient.c
The corresponding GSocket server test file naming doesn't conflict with other
class names, but rename it to socket-testserver.c for consistency.

Closes #2855

gio/tests/meson.build

@@ -590,8 +590,8 @@ test_extra_programs += {
   'proxy' : {'install' : false},
   'resolver' : {'install' : false},
   'send-data' : {'install' : false},
-  'socket-server' : {'install' : false},
-  'socket-client' : {
+  'socket-testserver' : {'install' : false},
+  'socket-testclient' : {
     'extra_sources' : ['gtlsconsoleinteraction.c'],
     'install' : false,
   },

gio/tests/socket-common.c

@@ -1,4 +1,4 @@
-/* #included into both socket-client.c and socket-server.c */
+/* #included into both socket-testclient.c and socket-testserver.c */
 
 #ifdef G_OS_UNIX
 static const char *unix_socket_address_types[] = {