tests on cran take too long

[mb706]

We should try to reduce CRAN tests to those that interact with non-mlr3-ecosystem packages (so their maintainers have to give us prior notice about breaking mlr3pipelines). mlr3-internal things are checked by our CI setup already.

Currently we are at 13 minutes and CRAN wants us to be < 10. The run log breaks it down into

• checking R code for possible problems ... [99s] OK
• running examples for arch 'i386' ... [76s] OK
• running examples for arch 'x64' ... [91s] OK
• running tests for arch 'i386' ... [170s] OK
• running tests for arch 'x64' ... [185s] OK
• everything else probably takes about 2-3 minutes.

x64 and i386 both get tested, so we aim at < 4 min for test + examples for each architecture.