Add additional test for multiple repr (#3)

Signed-off-by: Ahmed Abdelraoof <[email protected]>

safe-discriminant/tests/fail/double_repr.rs

@@ -0,0 +1,11 @@
+use safe_discriminant::Discriminant;
+
+#[derive(Discriminant)]
+#[repr(u8)]
+#[repr(u16)]
+enum Foo {
+    A = 1,
+    B = 2,
+}
+
+fn main() {}

safe-discriminant/tests/fail/double_repr.stderr

@@ -0,0 +1,17 @@
+error: conflicting representation hints
+ --> tests/fail/double_repr.rs:5:8
+  |
+5 | #[repr(u16)]
+  |        ^^^
+
+error[E0566]: conflicting representation hints
+ --> tests/fail/double_repr.rs:4:8
+  |
+4 | #[repr(u8)]
+  |        ^^
+5 | #[repr(u16)]
+  |        ^^^
+  |
+  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+  = note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
+  = note: `#[deny(conflicting_repr_hints)]` on by default