htmldocck: Don't silently ignore `!@has` comments

[camelid]

Currently, htmldocck (the script that runs src/test/rustdoc tests) has two
primary test assertions: @has and @!has. @has checks for the presence of
text in tags selected with an XPath selector. @!has does the same, except it
checks for the absence of text. However, it is very easy to write !@has
instead of @!has by mistake (see #80565 for an example of some tests that used
!@has and had to be fixed), but !@has will be silently ignored by htmldocck.
Instead, htmldocck should either accept !@has or error out and cause the test
to fail. Otherwise, it's confusing to people who are new to htmldocck and, even
worse, we will likely end up with tests that have silently-ignored assertions.

cc @GuillaumeGomez