suzerain: #2980 capture challenging data

Breaks with
../../sz/tests/test_bl.cpp(264): fatal error in "find_edge": critical check SUZERAIN_SUCCESS == suzerain_bl_find_edge( coeffs_H0, &location, dB.get(), b.bw, b.dbw) failed [0 != -2]
which is interesting as -2 is the code for SUZERAIN_CONTINUE indicating
that the call the suzerain_bspline_crossing likely couldn't reach
the requested GSL_DBL_EPSILON tolerance in the provide number of
iterations.

Investigating...

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42275 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2980 avoid passing NaN to linear_combination

This defensively works around the source of the crash giving
rise to ticket #2980.  It does not, however, address the source
of the NaN.  That's for another commit.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42276 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2980 improve test quality

Ensures that a finite result was found for the challenging data

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42277 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2981 drop tol for ChallengingFixture in test_bl

The root cause of #2980 was that *lower and *upper differed
by more than macheps in a way that couldn't be resolved in
any number of iterations.  Loosen the tolerance a touch.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42278 0bd85f0b-c151-43b9-a1e9-27231104551f