simplify test for em strong

tests/data/em_strong.html

-<p><strong><em>em strong</em></strong></p>
-<p><strong><em>one</em> at the start</strong></p>
-<p><strong>one at the <em>end</em></strong></p>
-<p><strong>one <em>in the</em> middle</strong></p>
-<p><strong>one with <em>asterisks</em></strong></p>
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+<p><strong><em>em strong</em> strong</strong></p>
+<p><strong>strong <em>em strong</em></strong></p>
+<p><strong>strong <em>em strong</em> strong</strong></p>
+<p><strong>strong <em>em strong</em></strong></p>
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tests/data/em_strong.md

-___em strong___
+___em strong_ strong__
 
-___one_ at the start__
+__strong _em strong___
 
-__one at the _end___
+__strong _em strong_ strong__
 
-__one _in the_ middle__
-
-**one with *asterisks***
\ No newline at end of file
+**strong *em strong***
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