improve tests

tests/data/inline_link_title.html

+<p><a href="http://example.com" title="Title">single quotes</a> and <a href="http://example.com" title="Title">double quotes</a></p>
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tests/data/inline_link_title.md

+[single quotes](http://example.com 'Title') and [double quotes](http://example.com "Title")
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tests/data/reference_title.html

-<p><a href="http://example.com" title="Title">single quotes</a> and <a href="http://example.com" title="Title">double quotes</a></p>
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+<p><a href="http://example.com" title="example title">double quotes</a> and <a href="http://example.com" title="example title">single quotes</a> and <a href="http://example.com" title="example title">parentheses</a></p>
+<p>[invalid title]: <a href="http://example.com">http://example.com</a> example title</p>
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tests/data/reference_title.md

-[single quotes](http://example.com 'Title') and [double quotes](http://example.com "Title")
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+[double quotes] and [single quotes] and [parentheses]
+
+[double quotes]: http://example.com "example title"
+[single quotes]: http://example.com 'example title'
+[parentheses]: http://example.com (example title)
+[invalid title]: http://example.com example title
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