Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

crates/nu-command/tests/commands/str_/collect.rs

@@ -0,0 +1,53 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn test_1() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo 1..5 | into string | str collect
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "12345");
+}
+
+#[test]
+fn test_2() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo [a b c d] | str collect "<sep>"
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "a<sep>b<sep>c<sep>d");
+}
+
+#[test]
+fn construct_a_path() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo [sample txt] | str collect "."
+        "#
+        )
+    );
+
+    assert_eq!(actual.out, "sample.txt");
+}
+
+#[test]
+fn sum_one_to_four() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        1..4 | each { $it } | into string | str collect "+" | math eval
+        "#
+        )
+    );
+
+    assert!(actual.out.contains("10"));
+}