consistently use "next solver" instead of "new solver"

tests/ui/traits/next-solver/overflow/exponential-trait-goals.rs

@@ -0,0 +1,19 @@
+// compile-flags: -Znext-solver
+
+trait Trait {}
+
+struct W<T>(T);
+
+impl<T, U> Trait for W<(W<T>, W<U>)>
+where
+    W<T>: Trait,
+    W<U>: Trait,
+{
+}
+
+fn impls<T: Trait>() {}
+
+fn main() {
+    impls::<W<_>>();
+    //~^ ERROR overflow evaluating the requirement `W<_>: Trait`
+}