Remove file I/O from tests that don't need it (#11182)

# Description

This PR implements modifications to command tests that write unnecessary
json and csv to disk then load it with open, by using nuon literals
instead.

- Fixes #7189



# User-Facing Changes
None

# Tests + Formatting
This only affects existing tests, which still pass.

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

@@ -1,33 +1,27 @@
-use nu_test_support::fs::Stub::{EmptyFile, FileWithContentToBeTrimmed};
+use nu_test_support::fs::Stub::EmptyFile;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, pipeline};
 
 #[test]
 fn splits() {
-    Playground::setup("split_by_test_1", |dirs, sandbox| {
-        sandbox.with_files(vec![FileWithContentToBeTrimmed(
-            "los_tres_caballeros.csv",
-            r#"
-                first_name,last_name,rusty_at,type
-                Andrés,Robalino,10/11/2013,A
-                JT,Turner,10/12/2013,B
-                Yehuda,Katz,10/11/2013,A
-            "#,
-        )]);
+    let sample = r#"
+                [[first_name, last_name, rusty_at, type];
+                 [Andrés, Robalino, "10/11/2013", A],
+                 [JT, Turner, "10/12/2013", B],
+                 [Yehuda, Katz, "10/11/2013", A]]
+            "#;
 
-        let actual = nu!(
-            cwd: dirs.test(), pipeline(
-            r#"
-                open los_tres_caballeros.csv
+    let actual = nu!(pipeline(&format!(
+        r#"
+                  {sample}
                 | group-by rusty_at
                 | split-by type
                 | get A."10/11/2013"
                 | length
             "#
-        ));
+    )));
 
-        assert_eq!(actual.out, "2");
-    })
+    assert_eq!(actual.out, "2");
 }
 
 #[test]