selftests: efivarfs: return Kselftest Skip code for skipped tests

selftests: efivarfs: return Kselftest Skip code for skipped tests

BugLink: https://bugs.launchpad.net/bugs/1809704
When efivarfs test is skipped because of unmet dependencies and/or
unsupported configuration, it returns 0 which is treated as a pass
by the Kselftest framework. This leads to false positive result even
when the test could not be run.

Change it to return kselftest skip code when a test gets skipped to
clearly report that the test could not be run.

Kselftest framework SKIP code is 4 and the framework prints appropriate
messages to indicate that the test is skipped.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
(cherry picked from commit 1f0ea95853bd55a1812f2903b2f776853069f21f)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>