Add a test for error_on_line_overflow_strings

tests/target/configs-error_on_line_overflow_strings-false.rs

@@ -0,0 +1,9 @@
+// rustfmt-error_on_line_overflow_strings: false
+// Suppress an error on line overflow strings.
+
+fn main() {
+    let x = "                                                                                                  ";
+    let a = "
+  
+";
+}