Loosen the root value test a little #535
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This addresses a recent test failure in polycool, found by property testing in CI. It changes only the tests.
For context, for arbitrary polynomials we test root-finding by checking that the polynomial evaluates to approximately zero at the putative root. This is easier than testing that the putative root is close to an actual root, because in order to do that we'd need to know what the actual root is. The issue is that if the polynomial has a very large derivative near the root, the value at the putative root could be far from zero even if the root is very close.
In the failing example, the derivative was more than 1e172 and the approximate root (at about 0.93) evaluated to less than 2e159. That's comfortably consistent with our target relative error of 1e-12 once the derivative is taken into account.