`Selector.containing` matches text anywhere in the `Single`, instead of only matching the candidate element

elm-html-test 5.2.0

The following test should fail because the text "outside" is not within the "button" tag. But it currently passes. (Though strangely, `Query.hasNot` with the same selectors also passes.)

https://ellie-app.com/KDBTJfMSp8a1
```elm
module Main exposing (main)

import Expect
import Html exposing (Html, text)
import Html.Attributes exposing (selected)
import Test exposing (..)
import Test.Html.Query as Query
import Test.Html.Selector as Selector exposing (attribute, containing, tag)

main : Html msg
main =
Html.div []
[ text "outside"
, Html.button [] [ Html.text "Button" ]
]
|> Query.fromHtml
|> Query.has
[ tag "button"
, containing [ Selector.text "outside" ]
]
|> toString
|> Html.text
```

Reproduce the issue with the Elm example in the report, then locate the implementation of Selector.containing and its use through Query.has. Check the behavior against the provided button and outside text, and consider the issue done when the selector no longer matches text outside the candidate element and the example fails as expected.

Written by the indexing model from the issue text.