tests: add a missing shared map test (#19322)

vlib/v/tests/shared_map_ptr_test.v

@@ -0,0 +1,23 @@
+struct Abc {
+	f shared map[string]&Abc
+}
+
+fn test_main() {
+	c := Abc{}
+	b := Abc{}
+	a := Abc{
+		f: b.f
+	}
+
+	lock a.f, b.f {
+		a.f['a'] = &c
+	}
+	lock b.f {
+		b.f['b'] = &c
+	}
+
+	dump(b.f)
+	rlock b.f {
+		assert b.f.len == 2
+	}
+}