Use login form in tests checking expired passwords

The controller provided by the `devise-security` gem which tests
password is expired does not execute the `before_action` we have in our
application controller. That means it doesn't set the current locale.

We were having issues in the tests checking this behavior if the
previous test had set the current locale to a different one. This meant
the process running the browser had one locale while the process running
the test had a different one, which resulted in a page in English (as
expected), only the flash message notifying users their password expired
was in a different language.

To reproduce this behavior, run:

```
rspec './spec/system/welcome_spec.rb[1:1:2:2:1]' spec/system/users_auth_spec.rb:623 --order defined
```

I'm not sure whether this is a bug or it's a problem with the tests. In
theory it might be possible to reproduce a similar behavior in
production due to what we mention about the controller not executing the
`set_current_locale` method. But I haven't been able to reproduce the
situation, particularly since the password expiration seems to be
checked exclusively at login time (that is, if you stay logged in for 10
years, your password doesn't seem to expire).

So for now I'm just making the tests pass by using the login form
instead of using `login_as`.

spec/system/users_auth_spec.rb

@@ -580,11 +580,13 @@ describe "Users" do
   end
 
   scenario "Sign in, admin with password expired" do
-    user = create(:user, password_changed_at: Time.current - 1.year)
-    admin = create(:administrator, user: user)
+    user = create(:administrator).user
+    user.update!(password_changed_at: Time.current - 1.year)
 
-    login_as(admin.user)
-    visit root_path
+    visit new_user_session_path
+    fill_in "Email or username", with: user.email
+    fill_in "Password", with: user.password
+    click_button "Enter"
 
     expect(page).to have_content "Your password is expired"
 
@@ -617,11 +619,13 @@ describe "Users" do
   end
 
   scenario "Admin with password expired trying to use same password" do
-    user = create(:user, password_changed_at: Time.current - 1.year, password: "123456789")
-    admin = create(:administrator, user: user)
+    user = create(:administrator).user
+    user.update!(password_changed_at: Time.current - 1.year, password: "123456789")
 
-    login_as(admin.user)
-    visit root_path
+    visit new_user_session_path
+    fill_in "Email or username", with: user.email
+    fill_in "Password", with: user.password
+    click_button "Enter"
 
     expect(page).to have_content "Your password is expired"