TST: relax bounds of interpolate test to accomodate rounding error

A small rounding error appears with gcc-4.9 on i586 which causes the a
root to be -1e-19 instead of 0. and subsequently being cut by the exact
masking causing a testfailure.
Merge pull request #3907 from juliantaylor/interpolate-rounding

TST: interpolate: relax bounds of interpolate test to accomodate rounding error