test_tree: Move test_unicode_char_* to use a different UI element

In gedit 3.35, the "Open" button was re-designed to no longer have the
"Other Documents…" button inside it.

Move to testing the (hamburger) menu button instead, as it has an item
"Find and Replace…" which we can look for instead. This works on
pre-3.35 gedit too.
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