Describe the bug

unit tests should always assert that the results of the API they test are correct. This is not the case of the following:

• coordinate_transform_toy.cu
• trajectory_derive_toy.cu
• trajectory_distance_and_speed_toy.cu
• trajectory_spatial_bounds_toy.cu
• trajectory_subset_toy.cu

Expected behavior
Compute expected results and compare using GTEST / GMOCK assertions / expectations.

Note we should also remove the "_toy" suffix on these tests and rename them to test_<api name>.cpp (or .cu as necessary).

There's also no reason to only have one test (TEST_F or TYPED_TEST) per API. There should be plenty of tests, exercising different situations, such as mal-formed inputs, inputs with/without nulls, empty inputs, empty results, etc.

See RAPIDS cuDF cpp/tests for lots of good examples.