Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

crates/nu-command/tests/commands/mod.rs

@@ -0,0 +1,66 @@
+mod all;
+mod any;
+mod append;
+mod cal;
+mod cd;
+mod compact;
+mod cp;
+mod def;
+mod default;
+mod drop;
+mod each;
+mod echo;
+mod empty;
+mod enter;
+mod every;
+mod find;
+mod first;
+mod flatten;
+mod format;
+mod get;
+mod group_by;
+mod hash_;
+mod headers;
+mod help;
+mod histogram;
+mod into_filesize;
+mod into_int;
+mod keep;
+mod last;
+mod length;
+mod lines;
+mod ls;
+mod math;
+mod merge;
+mod mkdir;
+mod move_;
+mod open;
+mod parse;
+mod path;
+mod prepend;
+mod random;
+mod range;
+mod reduce;
+mod rename;
+mod reverse;
+mod rm;
+mod roll;
+mod rotate;
+mod save;
+mod select;
+mod semicolon;
+mod skip;
+mod sort_by;
+mod source;
+mod split_by;
+mod split_column;
+mod split_row;
+mod str_;
+mod touch;
+mod uniq;
+mod update;
+mod where_;
+mod which;
+mod with_env;
+mod wrap;
+mod zip;