When a test method name includes a slash (e.g. `test "signup on the
/signup page"`) the screenshot is generated in the nested directory on
systems that use slash as a directory separator (e.g. a screenshot
called `signup_page.png` is generated within `failures_signup_on_the_`).

Nesting screenshots causes an issue with `tmp:clear` rake task:

```
== Removing old logs and tempfiles ==
rails aborted!
Errno::EISDIR: Is a directory @ apply2files - tmp/screenshots/failures_signup_on_the_
/var/lib/gems/2.5.0/gems/railties-5.2.3/lib/rails/tasks/tmp.rake:41:in `block (3 levels) in <top (required)>'
/var/lib/gems/2.5.0/gems/railties-5.2.3/lib/rails/commands/rake/rake_command.rb:23:in `block in perform'
...
Tasks: TOP => tmp:clear => tmp:screenshots:clear
```

While the error could be prevented by changing `tmp:clear` task, there's
no reason to generate deep directory structures for tests using slashes.

To prevent a similar problem on Windows, we'll also "sanitize"
backslashes.

Replacing the problamatic characters with dashes seems to be a safe
workaround, although dash is very arbitrary choice in this case.
Co-Authored-By: NLouis-Michel Couture <louim_1@hotmail.com>