TST: add tests for ext parameter of {Interpolated,LSQ}UnivariateSpline

Light smoke tests only.
ENH: interpolate: string keywords to control extrapolation in fitpack2

splev accepts ext=0, 1, 2; class-based wrappers take `ext` as either
int, or a string value.
Merge pull request #2 from ev-br/pr-3773_

Pr 3773