ENH: add unit tests

[mathurinm]

some tests checking convergence through duality gap on toy problems would be a good start @QB3

[agramfort]

+1

[mathurinm]

https://github.com/QB3/CLaR/blob/master/clar/tests/test_clar.py#L20 we can probably make this clearer @QB3

[mathurinm]

https://github.com/QB3/CLaR/blob/master/clar/tests/test_mrce.py#L8 there is no assert

https://github.com/QB3/CLaR/blob/master/clar/tests/test_mrce.py#L55 there is no assert either

https://github.com/QB3/CLaR/blob/master/clar/tests/test_sgcl.py#L15 some of these parameters should be removed, the test should work with default values of solver()