This commit ignores the sqlite3 database file under the `activerecord/test/storage/`.

- Steps to reproduce

```ruby
cd activerecord
bundle update sdoc --conservative # not relevant to this issue, just to update sdoc
bin/test test/cases/schema_dumper_test.rb -n test_do_not_dump_foreign_keys_when_bypassed_by_config
git status
```

- Without this fix, `git status` shows `test/storage/` as `Untracked files`.
Here `Gemfile.lock` difference is due to `bundle update sdoc --conservative`

```ruby
$ git status
On branch main
You are currently bisecting, started from branch 'main'.
  (use "git bisect reset" to get back to the original branch)

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
  modified:   ../Gemfile.lock

Untracked files:
  (use "git add <file>..." to include in what will be committed)
  test/storage/

no changes added to commit (use "git add" and/or "git commit -a")
$
```

- With this fix, `git status` does not show any `Untracked files`.
```
$ git status
On branch add_gitignore_test_storage
You are currently bisecting, started from branch 'main'.
  (use "git bisect reset" to get back to the original branch)

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   ../Gemfile.lock

no changes added to commit (use "git add" and/or "git commit -a")
$
```