nice 2.18 regression tests

[Keith OHara]

Not a bug, but probably of interest here.

For some reason I compared the regression tests between 2.16 and 2.18, up 
through lyrics-tenor-clef.ly before I got tired.  Many look a bit nicer. For 
most changes I recognized the cause.

Some non-obvious changes:
‘ambitus-with-ligature.ly’ lost barlines, which looks correct

‘chord-tremolo-accidental.ly’ obviously fails.  The patch that introduced this 
feature didn't work well, so we removed it, but David wanted to keep the 
regression test in place.

‘font-kern.ly’ still broken on Windows, but maybe some progress thanks to help 
from Pango people

‘fret-diagrams-dots.ly’ ‘fret-diagrams-fingering.ly’ I happen to think the 
'skyline-integral' feature of fitting things together based on their outlines, 
might better be turned off for fret charts.

'lesNereides.ly' collides again.  The collision is with a cross-staff slur, 
which LilyPond has never so far included in collision-avoidance between 
systems.   However, we did have 'horizontal-padding' settings so that the 
'm.g.' from the lower system stayed clear of the low notes and 'Ped.' in the 
upper system.  Maybe these went awry.

I suspect that the automatic tester skips over full-page regression tests, at 
least in the image-comparison step, so developers should take a look at these 
independently, when a change might affect them.

Then I quit before test beginning with 'm'.