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/files.lua

@@ -471,12 +471,14 @@ do print("testing flush")
   assert(io.flush())        -- write to device
   assert(f:close())
 
-  local f = io.output("/dev/full")
-  assert(f:write("abcd"))   -- write to buffer
-  assert(not f:flush())     -- cannot write to device
-  assert(f:write("abcd"))   -- write to buffer
-  assert(not io.flush())    -- cannot write to device
-  assert(f:close())
+  local ok, f = pcall(io.output, "/dev/full")
+  if ok then  -- /dev/full exists (Linux); skip on macOS
+    assert(f:write("abcd"))   -- write to buffer
+    assert(not f:flush())     -- cannot write to device
+    assert(f:write("abcd"))   -- write to buffer
+    assert(not io.flush())    -- cannot write to device
+    assert(f:close())
+  end
 end