Fix input/regression/version-seen.ly for make check
Output from make check
was showing diffs for
input/regression/version-seen.ly at every new version. The problem was
that lilypond-book-preamble.ly ensures snippets are not required to
declare a \version, leading to the warning not being emitted and this
test failing. Therefore, version-seen must be redefined to #f.
Closes #6041 (closed).