Add tests for `multiset_solver`.

tests/multiset_solver.v

+From stdpp Require Import gmultiset.
+
+Section test.
+  Context `{Countable A}.
+  Implicit Types x y : A.
+  Implicit Types X Y : gmultiset A.
+
+  Lemma test1 x y X : {[x]} ⊎ ({[y]} ⊎ X) ≠ ∅.
+  Proof. multiset_solver. Qed.
+  Lemma test2 x y X : {[x]} ⊎ ({[y]} ⊎ X) = {[y]} ⊎ ({[x]} ⊎ X).
+  Proof. multiset_solver. Qed.
+  Lemma test3 x : {[x]} ⊎ ∅ ≠@{gmultiset A} ∅.
+  Proof. multiset_solver. Qed.
+  Lemma test4 x y z X Y :
+    {[z]} ⊎ X = {[y]} ⊎ Y →
+    {[x]} ⊎ ({[z]} ⊎ X) = {[y]} ⊎ ({[x]} ⊎ Y).
+  Proof. multiset_solver. Qed.
+End test.