What extra configuration tests are needed?

[Arlie-Capps]

Issue #1719 shows that Danny Taller ran into a problem by turning Sidre off. Maybe we need to test configurations with a few components turned off. What do we do now? What tests, if any, should we add? We all uneasily note that there is a combinatorial explosion lurking here. Perhaps we should test other common situations such as MPI off, RAJA and Umpire present or not, etc.

Also, should we expose common configurations as Spack variants?