Add test of underscore variable in format string

tests/ui/display-underscore.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("{_}")]
+pub struct Error;
+
+fn main() {}

tests/ui/display-underscore.stderr

@@ -0,0 +1,7 @@
+error: invalid format string: invalid argument name `_`
+ --> tests/ui/display-underscore.rs:4:11
+  |
+4 | #[error("{_}")]
+  |           ^ invalid argument name in format string
+  |
+  = note: argument name cannot be a single underscore