Add do and wait for n messages selenium

What does this MR do?

This MR adds the doAndWaitForNUIChanges and doAndWaitForNUIEvents functions.

Unlike doAndWaitForUIchange and doAndWaitForUIEvent, they allow to wait for multiple UI events/changes. These functions are now defined in terms of doAndWaitForNUIChanges and doAndWaitForNUIEvents respectively.

Related issues

N/A.

Changes to public APIs

doAndWaitForNUIChanges :: !Int !(JS TestState JSVal) -> JS TestState JSVal

doAndWaitForNUIEvents :: !Int !(JS TestState JSVal) -> JS TestState JSVal

are exported in the iTasks.Testing.Selenium.Interface module.

Author's checklist (required)

See CONTRIBUTING.md for the rationale behind these items:

• The commit history does not contain merges (use git rebase -i master if it does)
• Intermediate commits compile (use git rebase -i master if not)
• Newly added code follows the Platform code style
• Newly added code is documented
• If bugs have been solved, tests have been added
• Appropriate types have been used, especially in APIs
• If efficiency is part of the acceptance criteria of the issue, a benchmark is provided
• A changelog entry has been added if required. See CONTRIBUTING.md