Allow mesh to start at X0 or Y0

[reemo3dp]

My bed mesh starts at xmin = 0, which led to the X portion of the move to not be executed.

To reproduce:

[probe]
x_offset: 25
y_offset: 0

[bed_mesh]
mesh_min: 25,10
mesh_max: 110, 110

https://github.com/sporkus/probe_accuracy_tests/blob/master/probe_accuracy_test_suite.py#L323 will resolve to xmin = 0, leading to https://github.com/sporkus/probe_accuracy_tests/blob/master/probe_accuracy_test_suite.py#L249 to be evaluated to False

Thank you very much for these tools!