(#363): Fix whitespace removal around <tspan> elements

When there is something like

  <text>hello <tspan>world</tspan></text>

librsvg would trim away the space after "hello", so the rendered text
looked like "helloworld".

Now we only trim the ends of NodeChars elements if they don't have an
adjacent sibling; in the example above, the tspan is the sibling to
the right, and so the rightmost space in the "hello " NodeChars
doesn't get trimmed.

This makes the rendering in 340047.svg match Firefox; regenerated the
test reference file for that, and made it always use a sans-serif
font.

Fixes GNOME/librsvg#363
3 jobs from space-in-text-element in 8 minutes and 6 seconds (queued for 5 minutes and 25 seconds)
Status Job ID Name Coverage
  Test
passed #131234
fedora test

00:07:44

 
  Lint
manual #131236
allowed to fail manual
clippy
passed #131235
rustfmt

00:00:21