I have tests for my application. If those pass, I don't care if the individual module tests pass. I'm installing modules that I know on an OS that I have used them on before, so I don't consider it risky.
But there's another issue too -- some modules have tests that are not meant to be run when installing. These are tests that the developer wrote for development use and that will not work on other machines. So, the assumption that tests should be run on install is not shared by all module authors.