Revert some recent test changes

Revert some recent test changes

These should not have been done without discussion.

- Removal of the dcolumn table in 1a8b74f5e1c. Even if LyX does not support
dcolumn anymore, it is still a useful test whether tex2lyx imports it
correctly.

- Removal of the first "%% LyX" line. A long time ago it was decided (after
long discussion between at least Jean-Marc, Uwe and me), that this line is
interpreted by tex2lyx, and used to remove some LyX-generated preamble code.
These lines in the current tests exist on purpose (one can see in the diff
how the removal added unwanted stuff). I do not really like the
interpretation of the "%% LyX" line, but if this behaviour is to be changed
then this needs discussion first.

- Changed comment of \date. The comment was put there on purpose, and the
warning which was "fixed" by the change hints at a limitation in LyX, not a
tex2lyx problem (LyX does not know that a comment inset between some title
insets is OK). The roundtrip .tex output was OK with the old version.

- Change of \verbatiminput{foo}. This was supposed to test whether a
verbatim inset is correctly created even if the included file does not
exist.

- Removal of \lyxlines. Although these tests test input of files created by
old LyX versions, they are useful.

- Change of the lemma in test-modules.tex. The old version was put there on
purpose, and the file itself explains why it is translated to ERT.