Fixed failing conflict resolve spec

What does this MR do?

Fixes failing specs in master. This issue is caused by there not being enough room in the DOM to display all the content. Rather than free up space by adding more CSS, i've just made it toggle the tree list, which isn't needed by the test, this gives us more space in the window.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9645

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
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