Exclude the test kernel.

This workarounds a problem with clippy:
```
Error:   --> tests/test_kernels/stack_address/src/bin/basic_boot.rs:27:1
   |
27 | / fn panic(info: &core::panic::PanicInfo) -> ! {
28 | |     use core::fmt::Write;
29 | |
30 | |     let _ = writeln!(test_kernel_stack_address::serial(), "PANIC: {info}");
31 | |     exit_qemu(QemuExitCode::Failed);
32 | | }
   | |_^
   |
   = note: the lang item is first defined in crate `std` (which `bitflags` depends on)
   = note: first definition in `std` loaded from /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-dd28cbf307893515.rmeta
   = note: second definition in the local crate (`basic_boot`)
```

Author: jarovo

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - uses: r7kamura/rust-problem-matchers@v1.1.0
       - name: "Run `cargo check`"
-        run: cargo check --all-targets --all
+        run: cargo check --all-targets --all --exclude 'test_kernel*'  # Don't fail on panic_handler doubled definition.
       - name: "Check test kernels"
         run: cargo check --all
         working-directory: tests/test_kernels
@@ -123,7 +123,7 @@ jobs:
       - run: cargo --version --verbose
       - uses: Swatinem/rust-cache@v2
       - uses: r7kamura/rust-problem-matchers@v1.1.0
-      - run: cargo clippy --all --all-targets
+      - run: cargo clippy --all --all-targets  --exclude 'test_kernel*' # Don't fail on panic_handler doubled definition.
       - run: cargo clippy --all
         working-directory: tests/test_kernels