MDL-77365 count_words: Make span> an inline tag

This commit merges the fix and test for the incorrect counting of
words in essay type questions. The issue is due <span> tags not being
counted as inline tags.

lib/moodlelib.php

@@ -8386,7 +8386,7 @@ function count_words($string) {
                 </                              # Start of close tag.
                 (?!                             # Do not match any of these specific close tag names.
                     a> | b> | del> | em> | i> |
-                    ins> | s> | small> |
+                    ins> | s> | small> | span> |
                     strong> | sub> | sup> | u>
                 )
                 \w+                             # But, apart from those execptions, match any tag name.

lib/tests/moodlelib_test.php

@@ -4003,6 +4003,7 @@ EOF;
             [2, "one\ftwo"],
             [1, "SO<sub>4</sub><sup>2-</sup>"],
             [6, '4+4=8 i.e. O(1) a,b,c,d I’m black&blue_really'],
+            [1, '<span>a</span><span>b</span>'],
         ];
     }