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::Thread`.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/all/aDC9y829vZZBzZ2p@google.com
Link: https://github.com/Rust-for-Linux/pin-init/pull/58/commits/b004dd8e64d4cbe219a4eff0d25f0a5f5bc750ca


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>