test/CMakeLists.txt: Skip some tests when the machine has a single CPU

[sanvila]

Description

When the machine has a single CPU, skip three tests which are known not to work

Fixes #1557

Type of change

• [X] bug fix - change that fixes an issue
• [X] tests - change in tests

Tests

Note: I don't really know if you can test this easily (I mean, via CI/CD). I have already tested that the skipped tests make the machine to hang when it has a single CPU, so I'm marking "not needed" here.

• [X] not needed

Documentation

• [X] not needed

Breaks backward compatibility

• [x] No

Notify the following users

@pavelkumbrasev @dnmokhov @kboyarinov

Other information

I've marked "Allow edits by maintainers", so please do not hesitate to do that if it's useful/required.

Thanks.