tests: Unify whitespace visualization

This part of the code should be using the visualizeWhitespace(), too.

Change-Id: Id6c0f7579fa3092d7ef102045d3fdca707b3eb0b
HTML: Use the regular expression for determining quote level

We have that regexp, so let's make use of it. It will come handy later
on when we add support for spaces within the quote prefix regular
expression..

Change-Id: I6239d3dfccb7fcb2cdafa4158fcda184d58c0402
HTML: Accept quote marks interleaved with spaces in non-flowed mode

Since df8bff6e94273e5a6fb6bd3c018606f85b66dc6f, we employed a strict
algorithm which would ignore any quote level indication after a space
gets hit. While this is the right thing to do when parsing
format=flowed, it turns out that some MUAs out there not only have
user-configurable quote marks, but also default to *always* inserting
spaces between them -- KMail is one example, see the "KMail's quoting of
text in replies and format=flowed " thread at [1].

I believe that format=flowed is ultimately the only reasonable
specification of how to do quotes, and I would like to have a word with
people who think that replying with e.g. "|" as a quote separator. I am
not willing to go as far as to support *these* crazy separators, but in
order to better support these legacy clients which at least know ">",
but throw in an occasional space, this patch changes the code so that it
again recognizes this as a quote separator.

There's of course a downside to this as well -- older versions of
mailman would happily strip out format=flowed designation from the MIME
parts, and this means that Trojita will treat a line which is correctly
escaped in conformance with RFC3676, but which went through old mailman
by accident, and happens to start with an actual ">" as its content (and
not quote separator), in a wrong way.

[1] http://web.archiveorange.com/archive/v/vFLZWeVHCaFhIXmLa4oL

Change-Id: Id433b7f2a34e5f2f3f7f7d7ebccd78572e4db4fa