be more liberal in strikethru regression

Author: kivikakk

test/regression.txt

@@ -165,9 +165,9 @@ cmark-gfm strikethrough rules
 ````````````````````````````````
 
 ```````````````````````````````` example strikethrough
-This ~text~~~~ is ~~~~curious~.
+This ~text~ ~~is~~ ~~~curious~~~.
 .
-<p>This <del>text~~~~ is ~~~~curious</del>.</p>
+<p>This <del>text</del> <del>is</del> ~~~curious~~~.</p>
 ````````````````````````````````
 
 `~` should not be escaped in href — https://github.com/github/markup/issues/311