[test][math] Lower precision for polynomial test

When the roots are very close together the inversion becomes ill-conditioned.
Exclude these cases from the random generation, test some corner cases
with close roots and decrease the tolerance for root comparison.
4 jobs for !2671 with feature/relax-math-test-precision
latest detached
Status Name Job ID Coverage
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
manual minimal-dune-2.7-gcc
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