Add test for zero single char names

Author: marcin-serwin

tests/ui-toml/zero_single_char_names/clippy.toml

@@ -0,0 +1 @@
+single-char-binding-names-threshold = 0

tests/ui-toml/zero_single_char_names/zero_single_char_names.rs

@@ -0,0 +1,3 @@
+#![warn(clippy::many_single_char_names)]
+
+fn main() {}