new(ci): add a sample kmod source to test that we are able to build a kmod after kernel headers are installed

.github/workflows/Makefile

@@ -0,0 +1,9 @@
+MODULENAME=kmod
+
+obj-m += $(MODULENAME).o
+
+module:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
+
+clean:
+	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

.github/workflows/kmod.c

@@ -0,0 +1,16 @@
+#include <linux/module.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("test");
+
+static int devnode_dev_const_init(void)
+{
+	return 0;
+}
+
+static void devnode_dev_const_exit(void)
+{
+}
+
+module_init(devnode_dev_const_init);
+module_exit(devnode_dev_const_exit);

.github/workflows/test.yml

@@ -9,38 +9,40 @@ jobs:
   test-composite-action:
     runs-on: ubuntu-latest
     steps:
-      - name: Echo kernelrelease
-        run: uname -a
-
+      - uses: actions/checkout@v4
+
       - name: Test action
         uses: ./
         with:
           cache: 'true'
 
-      - name: Check kernel sources
-        run: ls -la /usr/src
+      - name: Check that a fake module can be built
+        working-directory: .github/workflows
+        run: make
 
   test-composite-action-cached:
     runs-on: ubuntu-latest
     needs: test-composite-action
     steps:
-      - name: Echo kernelrelease
-        run: uname -a
-
+      - uses: actions/checkout@v4
+
       - name: Store KERNEL_RELEASE
         shell: bash
         run: |
-          echo "KERNEL_RELEASE=$(uname -r)" >> $GITHUB_ENV  
-
-      - name: Check kernel sources (empty)
-        run: ls -la /usr/src
+          echo "KERNEL_RELEASE=$(uname -r)" >> $GITHUB_ENV
 
-      - name: Retrieve cached kernel sources
+      - name: Test that kernel sources are cached
         uses: actions/cache@v4
         with:
           path: |
             /usr/src/linux
-          key: ${{ runner.os }}-${{ runner.arch }}-${{env.KERNEL_RELEASE}}     
+          key: ${{ runner.os }}-${{ runner.arch }}-${{env.KERNEL_RELEASE}}
+
+      - name: Fully restore through action
+        uses: ./
+        with:
+          cache: 'true'
 
-      - name: Check kernel sources again
-        run: ls -la /usr/src   
+      - name: Check that a fake module can be built again
+        working-directory: .github/workflows
+        run: make