add decl safety tests

Author: lqd

tests/ui/decl_safety/safe_trait.🔬

@@ -0,0 +1,7 @@
+//@check-pass
+[
+    crate baguette {
+        safe trait Foo {}
+        safe impl Foo for u32 {}
+    }
+]

tests/ui/decl_safety/safe_trait_mismatch.stderr

@@ -0,0 +1,4 @@
+Error: check_trait_impl(unsafe impl Foo for u32 { })
+
+Caused by:
+    implementing the trait `Foo` is not unsafe

tests/ui/decl_safety/safe_trait_mismatch.🔬

@@ -0,0 +1,6 @@
+[
+    crate baguette {
+        trait Foo {}
+        unsafe impl Foo for u32 {}
+    }
+]

tests/ui/decl_safety/unsafe_trait.🔬

@@ -0,0 +1,7 @@
+//@check-pass
+[
+    crate baguette {
+        unsafe trait Foo {}
+        unsafe impl Foo for u32 {}
+    }
+]

tests/ui/decl_safety/unsafe_trait_mismatch.stderr

@@ -0,0 +1,4 @@
+Error: check_trait_impl(impl Foo for u32 { })
+
+Caused by:
+    the trait `Foo` requires an `unsafe impl` declaration

tests/ui/decl_safety/unsafe_trait_mismatch.🔬

@@ -0,0 +1,6 @@
+[
+    crate baguette {
+        unsafe trait Foo {}
+        impl Foo for u32 {}
+    }
+]