defmt-test: support the #[ignore] attribute & raise error on unknown attributes

[japaric]

The standard test runner uses this attribute to ignore (omit running) a unit test. We implement the same functionality or, at least, error when an unknown attribute is used within defmt_test::tests because right now cargo test-ing this:

#[ignore]
#[test]
fn loopback() { /* .. */ }

does run the test marked as #[ignore]

[japaric]

I did cargo expand and it looks like we are discarding all the attributes that appear in the input code.

[japaric]

This doesn't work either:

#[cfg(ignore)]
#[test]
fn loopback() { /* .. */ }

in the sense that the test is compiled and executed when --cfg ignore is not passed to rustc.

[jonas-schievink]

This now behaves as expected, do we want to close it, or leave it open to track #[ignore] support?

[japaric]

I'll open a new issue about the #[ignore] attribute.