attributes: add more compile fail tests

[japaric]

now that there's CI infrastructure in place for compile fail tests (added in #293) we should add tests for these scenarios:

• timestamp and panic_handler used on a function with the wrong signature. For instance, all these examples are UB (they are rejected at compile time today):

#[timestamp]
fn foo(x: u64) -> u64 { x + 1 }

#[panic_handler]
fn foo(x: bool) -> ! { info!("{:?}", x); loop {} }

#[panic_handler]
fn foo() {}

• global_logger used on a struct that has fields. I don't think this is much of an issue (unsure if it's even checked today) but the fields won't be accessed by defmt so accepting this would be misleading at best.

these compile fail tests are also a good place to check the helpfulness of our error messages. For example, this:

defmt::write!(42, "hello");

should say "error: expected defmt::Formatter, found i32", ideally with a span that points to the 42 expression, instead of the a helpful "method needs_tag not found on i32 value". (not sure what the current error message says)