tests: track the diagnostics of packed structs and field accessors

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

tests/ui/compile-fail/init/packed_struct.rs

@@ -0,0 +1,11 @@
+use pin_init::*;
+
+#[repr(C, packed)]
+struct Foo {
+    a: i8,
+    b: i32,
+}
+
+fn main() {
+    let _ = init!(Foo { a: -42, b: 42 });
+}

tests/ui/compile-fail/init/packed_struct.stderr

@@ -0,0 +1,10 @@
+error[E0793]: reference to field of packed struct is unaligned
+  --> tests/ui/compile-fail/init/packed_struct.rs:10:13
+   |
+10 |     let _ = init!(Foo { a: -42, b: 42 });
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this struct is 1-byte aligned, but the type of this field may require higher alignment
+   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
+   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
+   = note: this error originates in the macro `init` (in Nightly builds, run with -Z macro-backtrace for more info)