sort: generate test_start_buffer fixtures programmatically

tests/by-util/test_sort.rs

@@ -1909,7 +1909,22 @@ fn test_color_environment_variables() {
 #[test]
 fn test_start_buffer() {
     // Test that a file with the exact same size as the start buffer is handled correctly
-    test_helper("start_buffer_b", &["start_buffer_a.txt"]);
+    const FILE_B: &[u8] = &[b'b'; 8_000];
+    const FILE_A: &[u8] = b"aaa";
+
+    let mut expected = FILE_A.to_vec();
+    expected.push(b'\n');
+    expected.extend_from_slice(FILE_B);
+    expected.push(b'\n');
+
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.write_bytes("b", FILE_B);
+    at.write_bytes("a", FILE_A);
+
+    ucmd.args(&["b", "a"])
+        .succeeds()
+        .stdout_only_bytes(&expected);
 }
 
 /* spell-checker: enable */

tests/fixtures/sort/start_buffer_a.txt

@@ -1 +0,0 @@
-aaa