selftests/efivarfs: Add checking of the test return value

Add checking of the test return value, otherwise it will report success
forever for test_create_read().

Fixes: dff6d2ae ("selftests/efivarfs: clean up test files from test_create*()")
Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>