Bring over unit tests from `wp-cli/wp-cli`

[schlessera]

The framework package wp-cli/wp-cli still contains the unit tests for the WP_CLI\SearchReplacer class: https://github.com/wp-cli/wp-cli/blob/v1.5.1/tests/test-search-replace.php

These need to be brought over to this package instead.

[dugajean]

This has been fixed already @schlessera