join: implement option to suppress joined lines

tests/fixtures/join/suppress_joined.expected

@@ -0,0 +1,3 @@
+1 a
+8 h
+9 i

tests/test_join.rs

@@ -85,6 +85,18 @@ fn unpaired_lines() {
         .succeeds().stdout_only_fixture("unpaired_lines.expected");
 }
 
+#[test]
+fn suppress_joined() {
+    new_ucmd!()
+        .arg("fields_3.txt")
+        .arg("fields_2.txt")
+        .arg("-1")
+        .arg("2")
+        .arg("-v")
+        .arg("2")
+        .succeeds().stdout_only_fixture("suppress_joined.expected");
+}
+
 #[test]
 fn case_insensitive() {
     new_ucmd!()