cmake: fix kernel module installation

Install each module from its real build directory, propagate staged install roots, and fail when kbuild modules_install fails. Add a rootless CTest and CI regression for successful and failed installs.

Best practices followed:

Use the standard CMake DESTDIR and kbuild INSTALL_MOD_PATH staging interfaces:
https://cmake.org/cmake/help/latest/envvar/DESTDIR.html
https://docs.kernel.org/kbuild/modules.html#install-mod-path

Check child command status with execute_process RESULT_VARIABLE:
https://cmake.org/cmake/help/latest/command/execute_process.html

Register repeatable tests with CTest add_test:
https://cmake.org/cmake/help/latest/command/add_test.html

1 files changed, 20 insertions, 1 deletions

diff --git a/tests/README.md b/tests/README.md
index 689a16d8..65afd254 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -2,6 +2,25 @@
 
 These tests are done for Ubuntu and Debian distros. Please use latest stable Debian or Ubuntu to run the tests.
 
+## Kernel module installation test
+
+The kernel module installation regression test is rootless and does not need
+kernel headers. It uses a fake kernel build tree to verify that `ipoe`,
+`vlan_mon`, and `ppposeq` are installed from their actual build directories,
+honor `DESTDIR`, and make installation fail when `modules_install` fails.
+
+Run it through CTest after configuring the project:
+
+```bash
+ctest --test-dir build --output-on-failure -R '^kernel-module-install$'
+```
+
+It can also be run directly from the repository root:
+
+```bash
+sh tests/kernel-module-install/test.sh cmake
+```
+
 ## Preparations
 
 Install pytest
@@ -100,4 +119,4 @@ If you want to re-run tests 'from scratch', you may want to remove coverage data
 # from root dir (parent for this dir)
 sudo gcovr -d # build report and delete
 sudo gcovr -d # check that data is deleted (any coverage = 0%)
-```
\ No newline at end of file
+```