Expansion parses slices like "$PATH[1..2]", but so does "set" when assigning
"set PATH[1..2] . .". Commit be06f842a ("Allow to omit indices in index
range expansions") forgot the latter.
This one tests a bunch of separate stuff, so we put it into a few
different files.
The main, new one is "slices.fish", which tests various index expressions.