Add cfg(ktest) as a well-known configuration

osdk/src/commands/build/mod.rs

@@ -209,6 +209,9 @@ fn build_kernel_elf(
         // We do not really allow unwinding except for kernel testing. However, we need to specify
         // this to show backtraces when panicking.
         "-C panic=unwind",
+        // This is to let rustc know that "cfg(ktest)" is our well-known configuration.
+        // See the [Rust Blog](https://blog.rust-lang.org/2024/05/06/check-cfg.html) for details.
+        "--check-cfg cfg(ktest)",
     ]);
 
     if matches!(arch, Arch::X86_64) {