Make C++ tests in testsuite link-tests only

[dra27]

I've got some work-in-progress changes in dra27@0363e41 to test the runtime headers as part of the development-mode build, instead of in the testsuite (it tests each header individually, in both C and C++ pedantic mode). A bit like the config_main and config_boot test module (which catches forgetting to keep utils/config.fixed.ml and utils/config.generated.ml.in in sync), it has the benefit of catching these errors during development, which means they might get caught even before a PR is opened, or at least more quickly as mostly it means the build fails, rather than the testsuite. I've tested that commit on 4 reverts of the various fixes in: dra27#265, dra27#266, dra27#267 and dra27#268.

If we end up doing that, my suggestion is to keep both of the C++ tests in the testsuite, but use them as linking tests... i.e. I think your existing sockaddr compat test is obviously still worth running in C++ mode and I think it's worth having a general linking test for a C++ program testing both the bytecode and native linking.

I wasn't expecting it to end up this way, but at the moment the SARIF stuff does appear avoidable for now - but that led to wanting to share cxx.sh between both C++ tests, so I've proposed adding the slightly weird outputobj variable to ocamltest.