rust: pin-init: examples, tests: use `ignore` instead of conditionally compiling tests

Change `#[cfg(cond)]` to `#[cfg_attr(not(cond), ignore)]` on tests.

Ignoring tests instead of disabling them still makes them appear in the
test list, but with `ignored`. It also still compiles the code in those
cases.

Some tests still need to be ignore, because they use types that are not
present when the condition is false. For example the condition is
`feature = std` and then it uses `std:🧵:Thread`.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/all/aDC9y829vZZBzZ2p@google.com
Link: b004dd8e64
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://lore.kernel.org/all/20250605155258.573391-1-lossin@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/pin-init/examples/pthread_mutex.rs

@@ -139,7 +139,8 @@ mod pthread_mtx {
     }
 }
 
-#[cfg_attr(all(test, not(miri)), test)]
+#[cfg_attr(test, test)]
+#[cfg_attr(all(test, miri), ignore)]
 fn main() {
     #[cfg(all(any(feature = "std", feature = "alloc"), not(windows)))]
     {