[TEST] exception on illegal section names

Section names containing only a dot lead to an exception for wrong file
name. Make sure such section names are handles correctly.
[FIX] section names that lead to illegal file names for generate_html

Some section names lead to file names that are somehow illegal. One
example for this is a section name that contains just a dot. This leads
to the creation of a file for html generation that has the name
"section/.". Also, any section name containing a slash (/) would lead to
an illegal file name. This patch creates a file name for reports with
such section names that have dots and slashed substituted with the word
dot or slash respectively.
bugfix: add missing prereq module
[TEST] whether substituting illegal section names leaves legal ones intact
[FIX] make sure substituting illegal section names leave legal ones intact
[FIX] handling of empty Tapper-section headers

An empty Tapper-section header should be handled like a missing
one. That way we don't get another illegal file name (which would be an
empty string).
[TEST] for broken sections enhanced
[FIX] handle empty section names like missing section names

Empty section names lead to illegal file names for creating TAP
HTML. Also, they are of little use for the customer. Therefore, we now
handle them like missing section names.

Section names that only consist of whitespace are threated as empty.