tests/isclose: Use √ε where appropriate
tests/isclose: Use √ε where appropriate

We need √ε rather than ε in a couple of values that get squared; using ε, we
would get ε², which can underflow:

- The upper bound for rel_tol needs to be 1-√ε, as there are (implicit)
  dependencies on 1 - rel_tol².
- Similarly, using 1±√ε as the “fudge factor” in the positive and negative
  examples solves some issues with test flakyness.

ppb/ppb-vector,ppb/ppb-vector