Revisit: spectral/mandel: Disable skip-check again

[rolandtritsch]

spectral/mandel produces a different stderr every time it runs.

To fix/workaround this we enabled skip-check (in single-bench), when we run the tests/for all tests.

Skipping the checks for all tests is sub-optimal. Would suggest to revisit this.

One idea would be to make skipping the checks a parameter for single-bench and only skip the check for the tests that need to skip the check (right now spectral/mandel and real/compress).