Add trybuild crate to run ui tests

* add example file that should fail to compile due to an incompatible
  type (`() instead of `i32``)

firmware/defmt-test/Cargo.toml

@@ -24,3 +24,6 @@ defmt-info = []
 defmt-warn = []
 defmt-error = []
 default = ["defmt-trace"] # enable everything
+
+[dev-dependencies]
+trybuild = "1"

firmware/defmt-test/tests/ui.rs

@@ -0,0 +1,5 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

firmware/defmt-test/tests/ui/test-uses-invalid-type-annotation.rs

@@ -0,0 +1,10 @@
+fn main() {}
+
+#[defmt_test::tests]
+mod tests {
+    #[test]
+    #[ignore]
+    fn assert_eq() {
+        let x: () = 42;
+    }
+}