TST: improve parametrization style

[mdhaber]

Improves pytest output and make progress toward benchmarking by ensuring that lambdas have string names associated with them.

Now, the pytest output shows, e.g.

test_marray.py::test_arithmetic_unary[numpy-uint8-+x-<lambda>] 
test_marray.py::test_arithmetic_unary[numpy-uint8--x-<lambda>] 
test_marray.py::test_arithmetic_unary[numpy-uint8-abs-abs] 
test_marray.py::test_arithmetic_unary[numpy-uint8-x.__abs__-<lambda>] 
test_marray.py::test_arithmetic_unary[numpy-uint8-x.__neg__-<lambda>] 
test_marray.py::test_arithmetic_unary[numpy-uint8-x.__pos__-<lambda>] 

whereas before it was like:

test_marray.py::test_arithmetic_unary[numpy-uint8-<lambda>0] 
test_marray.py::test_arithmetic_unary[numpy-uint8-<lambda>1] 
test_marray.py::test_arithmetic_unary[numpy-uint8-<lambda>2] 
test_marray.py::test_arithmetic_unary[numpy-uint8-<lambda>3] 

[lucascolley]

nice!