gpa: Add never_unmap and retain_metadata test

lib/std/heap/general_purpose_allocator.zig

@@ -1458,3 +1458,19 @@ test "bug 9995 fix, large allocs count requested size not backing size" {
     buf = try allocator.realloc(buf, 2);
     try std.testing.expect(gpa.total_requested_bytes == 2);
 }
+
+test "retain metadata and never unmap" {
+    var gpa = std.heap.GeneralPurposeAllocator(.{
+        .safety = true,
+        .never_unmap = true,
+        .retain_metadata = true,
+    }){};
+    defer std.debug.assert(gpa.deinit() == .ok);
+    const allocator = gpa.allocator();
+
+    const alloc = try allocator.alloc(u8, 8);
+    allocator.free(alloc);
+
+    const alloc2 = try allocator.alloc(u8, 8);
+    allocator.free(alloc2);
+}