The diff patch file (attached as testcases.patch) contains the new testcases for the the previously untested gsl_interp_eval_integ() function of GSL. Consequently, these testcases increase the coverage of the regression testsuite by 1 new function and ~10 new lines. Furthermore, these tests have been extracted from a real-world client of GSL, hstaxehttps://github.com/spacetelescope/hstaxe/tree/main, and hence represents a real-world example of how this library is being used. Please do let me know if you have any comments/suggestions for improving this testcase contribution request.