Previously it wasn't possible to test something like this:
```
assert_select_jquery :remove, "[data-placeholder~=name]"
```
This commit fixes it by escaping `[` and `]` characters in the selector.
It appears that by the time this file is loaded, the `Rails::Dom::Testing...`
module has already been included into the relevant classes.
Because the ancestor chain is copied at include time, including a new module
into `Rails::Dom::Testing...` after it has been included has no effect on the
classes we wanted to patch, so just re-open the class here.
Inject into Rails::Dom::Testing::Assertions::DomAssertions
Explicit require statement increases the likelihood that further changes in
module names will be detected early.
Gem dependency on rails-dom-testing will ensure that this change is only
picked up with versions of Rails that are compatible with rails-dom-testing.
