[FEA] Update integration tests to compare result schemas between GPU and CPU

[andygrove]

Is your feature request related to a problem? Please describe.
Many of our integration tests run the same query on GPU then CPU and compare the results. It would be nice to also compare the result schema so that we can be sure that we return the same column names, types, and nullability flags.

Describe the solution you'd like
Update the assert_gpu_and_cpu_are_equal_* methods to compare schema as well as results. We should add an option to disable these checks since there may be edge cases that we won't support.

Describe alternatives you've considered
None

Additional context
N/A