Update test_common.cpp: use integer constant for knot index

[cedricpradalier]

Converted 1.0 and 0.0 to 1 and 0 respectively in the first argument of parent_T_spline(i,u) (and derivative): because this is a knot index argument. It adds nothing to the test itself and is confusing to the reader who wants to check how Sophus splines are used.

[strasdat]

thanks