testing: Avoid @import-ing builtins module

Use std.builtins instead.

lib/std/testing.zig

@@ -451,7 +451,7 @@ test {
 
 /// Given a type, reference all the declarations inside, so that the semantic analyzer sees them.
 pub fn refAllDecls(comptime T: type) void {
-    if (!@import("builtin").is_test) return;
+    if (!std.builtin.is_test) return;
     inline for (std.meta.declarations(T)) |decl| {
         _ = decl;
     }