ubuntu_kernel_selftests.cpu-hotplug fails with Bionic on single processor systems

Log from an ADT failure:
    amd64: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-bionic/bionic/amd64/l/linux-aws/20190215_172152_24535@/log.gz

When the test system has only one available CPU, the cpu-on-off-test.sh script used to return success:

22:14:42 DEBUG| [stdout] selftests: cpu-on-off-test.sh
22:14:42 DEBUG| [stdout] ========================================
22:14:42 DEBUG| [stdout] pid 30673's current affinity mask: 1
22:14:42 DEBUG| [stdout] pid 30673's new affinity mask: 1
22:14:42 DEBUG| [stdout] CPU online/offline summary:
22:14:42 DEBUG| [stdout] skip all tests:: since there is only one cpu: 0
22:14:42 DEBUG| [stdout] ok 1..1 selftests: cpu-on-off-test.sh [PASS]

However, after the script from the source package started to be replaced to fix bug 1809439 (commit https://kernel.ubuntu.com/git/ubuntu/autotest-client-tests.git/commit/?id=53ef6cf2cc35cbd13a16c363af05e22a0cbcbb75) it started to fail:

18:12:56 DEBUG| [stdout] selftests: cpu-on-off-test.sh
18:12:56 DEBUG| [stdout] ========================================
18:12:56 DEBUG| [stdout] pid 30620's current affinity mask: 1
18:12:56 DEBUG| [stdout] pid 30620's new affinity mask: 1
18:12:56 DEBUG| [stdout] CPU online/offline summary:
18:12:56 DEBUG| [stdout] skip all tests:: since there is only one cpu: 0
18:12:56 DEBUG| [stdout] not ok 1..1 selftests: cpu-on-off-test.sh [FAIL]

The failure is caused on Bionic by the fact that the new script returns ksft_skip=4 when the test cannot be performed, however this return code is not considered as SKIP on the older Kselftest framework, but as FAIL instead. For the same reason this also probably impacts older series as well.