test solve_ndisj a bit

tests/solve_ndisj.ref

+The command has indeed failed with message:
+Ltac call to "solve_ndisj" failed.
+No applicable tactic.

tests/solve_ndisj.v

+From stdpp Require Import namespaces strings.
+
+Lemma test1 (N1 N2 : namespace) :
+  N1 ## N2 → ↑N1 ⊆@{coPset} ⊤ ∖ ↑N2.
+Proof. solve_ndisj. Qed.
+
+Lemma test2 (N1 N2 : namespace) :
+  N1 ## N2 → ↑N1.@"x" ⊆@{coPset} ⊤ ∖ ↑N1.@"y" ∖ ↑N2.
+Proof. solve_ndisj. Qed.
+
+Lemma test3 (N : namespace) :
+  ⊤ ∖ ↑N ⊆@{coPset} ⊤ ∖ ↑N.@"x".
+Proof. solve_ndisj. Qed.
+
+Lemma test4 (N : namespace) :
+  ⊤ ∖ ↑N ⊆@{coPset} ⊤ ∖ ↑N.@"x" ∖ ↑N.@"y".
+Proof. solve_ndisj. Qed.
+
+Lemma test5 (N1 N2 : namespace) :
+  ⊤ ∖ ↑N1 ∖ ↑N2 ⊆@{coPset} ⊤ ∖ ↑N1.@"x" ∖ ↑N2 ∖ ↑N1.@"y".
+Proof.
+  Fail solve_ndisj. (* FIXME: it should be able to solve this. *)
+  apply namespace_subseteq_difference.
+  { apply ndot_preserve_disjoint_r. set_solver. }
+  solve_ndisj.
+Qed.