Update ksr system test to include server

Update ksr system test to include server

Prepare the system test for tests that require a server to import
created SKR files. This means the test script needs adjustments to
take into account the directory test files are located.

In addition, the check_keys function is renamed to ksr_check_keys
because the name clashes with check_keys from kasp.sh. It also has an
extra parameter added, offset, that can be used to check ksr files that
are created in the past or future.