Add ui test with suffixed string literals in attribute

Author: dtolnay

test_suite/tests/ui/malformed/str_suffix.rs

@@ -0,0 +1,10 @@
+use serde::Serialize;
+
+#[derive(Serialize)]
+#[serde(bound = ""huh)]
+pub struct Struct {
+    #[serde(rename = ""what)]
+    pub field: i32,
+}
+
+fn main() {}

test_suite/tests/ui/malformed/str_suffix.stderr

@@ -0,0 +1,11 @@
+error: unexpected suffix `huh` on string literal
+ --> tests/ui/malformed/str_suffix.rs:4:17
+  |
+4 | #[serde(bound = ""huh)]
+  |                 ^^^^^
+
+error: unexpected suffix `what` on string literal
+ --> tests/ui/malformed/str_suffix.rs:6:22
+  |
+6 |     #[serde(rename = ""what)]
+  |                      ^^^^^^