Update integration checks to use satijalab/seurat-ci Docker image

[dcollins15]

Picking up where #217 left off, this PR introduces two key improvements to the Integration Checks workflow:

1. Enables tests to be run via R CMD check by dropping the /tests directory from .Rbuildignore and then updating /test/testthat.R file to run the tests for SeuratObject rather than Seurat (not sure if this was intentional or not). Until now, R CMD check and rcmdcheck::rcmdcheck by extension have never been running SeuratObject’s tests, including during CRAN’s reverse dependency checks.

2. Updates the workflow to use the satijalab/seurat-ci Docker image as its runner environment, eliminating the need for the r-lib/actions we were previously using and (critically) speeding up the workflow.

The initial implementation of the Integration Checks workflow was taking ~55 minutes to run, owing mostly to dependency installation. Since the satijalab/seurat-ci image uses rocker/r2u as its base, package installation is very fast and the workflow now takes a little under 5 minutes to run 🚀 Another option to speed up dependency installation would be to use rspm which is provided as a built-in option for r-lib/actions/setup-r@v2 like we do here. This would have reduced the workflow runtime down to ~20 minutes. For the sake of posterity, I’ve included that change as b486bd5 so that implementation can always be recycled later.

Transitioning to a Docker-based action has the handy side-effect of introducing a portable testing environment that developers can use locally 🤘(more on this soon). The only real downside is that the r2u project only supports R-release so we're no longer able to run checks with R-oldrelease or R-devel.

This should probably wait for satijalab/seurat-docker#4 before being merged 😬

tl;dr: Updates the Integration Checks workflow, delivering an 11x speedup.