Removes CDPT (generic CDP)

CDPT used to be a generic so that we could inject Browser, Session, Page and
Client. At some point, it [thankfully] became a generic only to inject Client.

This commit removes the generic and bakes the *Server.Client instance in CDP.
It uses a socketpair for testing.

BrowserContext is still generic, but that's generic for a very different reason
and, while I'd like to remove that generic too, it belongs in a different PR.

src/testing.zig

@@ -445,10 +445,6 @@ pub fn pageTest(comptime test_file: []const u8) !*Page {
     return page;
 }
 
-test {
-    std.testing.refAllDecls(@This());
-}
-
 const log = @import("log.zig");
 const TestHTTPServer = @import("TestHTTPServer.zig");