Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

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

@@ -0,0 +1,34 @@
+mod basename;
+mod dirname;
+mod exists;
+mod expand;
+mod join;
+mod parse;
+mod split;
+mod type_;
+
+use std::path::MAIN_SEPARATOR;
+
+/// Helper function that joins string literals with '/' or '\', based on host OS
+fn join_path_sep(pieces: &[&str]) -> String {
+    let sep_string = String::from(MAIN_SEPARATOR);
+    pieces.join(&sep_string)
+}
+
+#[cfg(windows)]
+#[test]
+fn joins_path_on_windows() {
+    let pieces = ["sausage", "bacon", "spam"];
+    let actual = join_path_sep(&pieces);
+
+    assert_eq!(&actual, "sausage\\bacon\\spam");
+}
+
+#[cfg(not(windows))]
+#[test]
+fn joins_path_on_other_than_windows() {
+    let pieces = ["sausage", "bacon", "spam"];
+    let actual = join_path_sep(&pieces);
+
+    assert_eq!(&actual, "sausage/bacon/spam");
+}