test/cxx: Fix complex number comparisons

[raffenet]

Pull Request Description

C++20 removed support for == and != on complex numbers. Instead, compare the individual parts. Fixes compilation with Intel oneAPI C++ (icpx).

Author Checklist

• [x] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [x] Commits Follow Good Practice Commits are self-contained and do not do two things at once. Commit message is of the form: module: short description Commit message explains what's in the commit.
• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
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[raffenet]

test:mpich/custom compiler: intel-oneapi

[raffenet]

After some discussion with @jeffhammond and testing on https://godbolt.org/, the errors in Jenkins look to be an incompatibility between Intel oneAPI and the base system compiler (gcc 4.8.5). It doesn't hurt to add this change, but it's likely not needed in a supported installation.

[raffenet]

The new Jenkins build is here and has no issue with this test. Closing.