Add Tests for SetTimeLimit()

[kwesiRutledge]

It would be nice to add a simple test for SetTimeList() method of the gurobiSolver object.

One example of a similar test is TestEnv_GetTimeLimit1 from testing/gurobi/env_test.go. Feel free to expand the test to do more if you would like!

Suggested Final Product(s):

• The final tests should be written in gurobisolver_test.go following the format for test naming (i.e., TestGurobiSolver_SetTimeLimit#).