join: implement the -o option

tests/fixtures/join/autoformat.expected

@@ -0,0 +1,5 @@
+1 a a
+2 b b
+3 c d
+4 d g
+5 e i

tests/fixtures/join/different_lengths.txt

@@ -0,0 +1,5 @@
+1 a
+2 b c
+3 d e f
+4 g h
+5 i

tests/fixtures/join/unpaired_lines_format.expected

@@ -0,0 +1,6 @@
+   f 2 a
+   g 3 b
+   h 4 c
+   i 5 f
+   j 6 g
+   k 7 h

tests/test_join.rs

@@ -124,3 +124,52 @@ fn multitab_character() {
         .arg("э")
         .fails().stderr_is("join: error: multi-character tab э");
 }
+
+#[test]
+fn default_format() {
+    new_ucmd!()
+        .arg("fields_1.txt")
+        .arg("fields_2.txt")
+        .arg("-o")
+        .arg("1.1 2.2")
+        .succeeds().stdout_only_fixture("default.expected");
+
+    new_ucmd!()
+        .arg("fields_1.txt")
+        .arg("fields_2.txt")
+        .arg("-o")
+        .arg("0 2.2")
+        .succeeds().stdout_only_fixture("default.expected");
+}
+
+#[test]
+fn unpaired_lines_format() {
+    new_ucmd!()
+        .arg("fields_2.txt")
+        .arg("fields_3.txt")
+        .arg("-a")
+        .arg("2")
+        .arg("-o")
+        .arg("1.2 1.1 2.4 2.3 2.2 0")
+        .succeeds().stdout_only_fixture("unpaired_lines_format.expected");
+}
+
+#[test]
+fn autoformat() {
+    new_ucmd!()
+        .arg("fields_2.txt")
+        .arg("different_lengths.txt")
+        .arg("-o")
+        .arg("auto")
+        .succeeds().stdout_only_fixture("autoformat.expected");
+}
+
+#[test]
+fn empty_format() {
+    new_ucmd!()
+        .arg("fields_1.txt")
+        .arg("fields_2.txt")
+        .arg("-o")
+        .arg("")
+        .fails().stderr_is("join: error: invalid file number in field spec: ''");
+}