Move some experimental.regrid_bilinear... tests.

To help with work on the new `Cube.regrid()` interface I plan to submit a couple of PRs against the snappily named `iris.experimental.regrid.regrid_bilinear_rectilinear_src_and_grid()` function. *This* PR just moves some tests into the unit test area to grease those wheels.

NB. By switching from the `global_pp()` stock cube to `lat_lon_cube()` it removes the unnecessary dependence on test data.