This adds a test to the recent GetLayerName function, and moves
it to the base.h file for re-use for any code that needs a
layer name.
This test covers the case that caused lp:1824750 [1].
[1]: https://bugs.launchpad.net/kicad/+bug/1824750
The SEXPR class is a useful general-purpose S-Expression library
class and can (maybe) be used else where. It also should get
test coverage, as even if noone else uses it, it's critical for the
kicad2step exporter.
Also add some test coverage for some kicad2step routines. For now,
they're not useful outside kicad2step, but they are at least a useful
reference for S-Expression parsing.
John Beard