Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

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

@@ -0,0 +1,55 @@
+use nu_test_support::fs::Stub::EmptyFile;
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+// FIXME: jt: needs more work
+#[ignore]
+#[test]
+fn checks_if_existing_file_exists() {
+    Playground::setup("path_exists_1", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("spam.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo spam.txt | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}
+
+#[test]
+fn checks_if_missing_file_exists() {
+    Playground::setup("path_exists_2", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo spam.txt | path exists"
+        );
+
+        assert_eq!(actual.out, "false");
+    })
+}
+
+#[test]
+fn checks_if_dot_exists() {
+    Playground::setup("path_exists_3", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo '.' | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}
+
+#[test]
+fn checks_if_double_dot_exists() {
+    Playground::setup("path_exists_4", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo '..' | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}