Add lush feature tests and fix /dev/full test for macOS

Add test suite under testes/lush/ covering backtick commands, argv
parsing, ${} interpolation, and $NAME environment variables. Wire
them into testes/all.lua so they run with the full Lua 5.5 suite.

Skip /dev/full test in files.lua when the device doesn't exist
(macOS has no /dev/full).

testes/all.lua

@@ -199,6 +199,12 @@ dofile('bitwise.lua')
 assert(dofile('verybig.lua', true) == 10); collectgarbage()
 dofile('files.lua')
 
+-- lush shell feature tests
+dofile('lush/commands.lua')
+dofile('lush/argv.lua')
+dofile('lush/interpolation.lua')
+dofile('lush/envvars.lua')
+
 if #msgs > 0 then
   local m = table.concat(msgs, "\n  ")
   warn("#tests not performed:\n  ", m, "\n")