rustfmt test: add stable-only tests for cfg_select! formatting

These tests show that `cfg_select!` won't be formatted on `stable` or `beta`
release channels.

Author: ytmimi

src/tools/rustfmt/tests/source/cfg_select_stable.rs

@@ -0,0 +1,10 @@
+// rustfmt-stable: true
+
+// While we gate the `cfg_select!` formatting behind the `is_nightly_channel!()` check
+// this test helps ensure that we don't start formatting `cfg_select!` on the `stable`
+// or `beta` release channels. It is intentionally formatted incorrectly. As soon as the
+// `is_nightly_channel!()` gate is removed this will start formatting and we can remove this test.
+cfg_select! (
+       unix     => 1,
+       windows     => 1,
+);

src/tools/rustfmt/tests/target/cfg_select_stable.rs

@@ -0,0 +1,10 @@
+// rustfmt-stable: true
+
+// While we gate the `cfg_select!` formatting behind the `is_nightly_channel!()` check
+// this test helps ensure that we don't start formatting `cfg_select!` on the `stable`
+// or `beta` release channels. It is intentionally formatted incorrectly. As soon as the
+// `is_nightly_channel!()` gate is removed this will start formatting and we can remove this test.
+cfg_select! (
+       unix     => 1,
+       windows     => 1,
+);