add test for layer level encoding

mapserver/mapserver#4758
add test for 0-length legend name

mapserver/mapserver#4782
add empty raster query test

c.f. mapserver/mapserver#4778
fix encoding header
add test for edge case follow labels (mapserver/mapserver#4795)

these special cases can fall back to AUTO positionning:
- lines with two vertices
- text with 1, 2, or 3 letters
Add test for https://github.com/mapserver/mapserver/issues/4800
Add missing expected result (https://github.com/mapserver/mapserver/issues/4800)
Merge branch 'master' into wfs2