Add ui test of colliding From impls

tests/ui/same-from-type.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("failed to open")]
+    OpenFile(#[from] std::io::Error),
+    #[error("failed to close")]
+    CloseFIle(#[from] std::io::Error),
+}
+
+fn main() {}

tests/ui/same-from-type.stderr

@@ -0,0 +1,5 @@
+error: cannot derive From because another variant has the same source type
+ --> tests/ui/same-from-type.rs:8:15
+  |
+8 |     CloseFIle(#[from] std::io::Error),
+  |               ^^^^^^^^^^^^^^^^^^^^^^