Should place SDFormat files for integration tests in one location

## Desired behavior
To have a standard location for SDFormat files used in integration testing.

## Alternatives considered
n/a

## Implementation suggestion
I propose we move all SDFormat files used for integration tests into `test/sdf`. Currently, some files are in `test/sdf` and others are in `test/integration/`. This causes confusion as to where to place new SDFormat files when writing tests.

## Additional context
n/a

Compare the SDFormat files currently in test/sdf and test/integration, then inspect the integration tests that reference them. Move the integration-test files into test/sdf and update their references so the tests still locate them; done means all such files have one standard location and the integration tests pass.

Written by the indexing model from the issue text.