TST: avoid unused variable in assert

[minrk]

CHECK appears to be the way checks are done elsewhere, this was the only assert left in the file.

I assume assert is being optimized out, causing compiler error with -Werror=unused-variable. cpp tests pass during conda build with this patch.

First try at #3174 was wrong, but it didn't let me reopen that PR after I figured it out because I updated the branch while it was closed.

[garth-wells]

Thanks - clearly wrong to be using assert here.

[jhale]

@minrk could you help us get our compile flags tightened so this doesn't happen again?

I'm surprised -Wall doesn't include this one.

https://github.com/FEniCS/dolfinx/blob/main/cpp/test/CMakeLists.txt#L51

[minrk]

conda-forge adds -DNDEBUG to CPPFLAGS, which strips asserts. It would be reasonable to remove -DNDEBUG from the compile args in the test directory, I would imagine, which would also allow this to pass. I don't know the best cmake incantation to remove arguments.

If you know you don't want asserts, you could add -DNDEBUG by default as well, but I'm not sure that's the right way to do it, because other asserts that aren't the only use of variables would just get silently skipped. Seems like a linter ought to be the level of avoiding asserts, but I don't know the state of linters for C++.