Link all targets in a directory against Dune::Common

Summary

This is a follow-up MR about setting include-directories on the Dune::Common target, !1355 (merged). It is related to several other MRs in the core modules, e.g., dune-istl!598 (merged), dune-geometry!257 (merged), dune-grid!763 (merged), dune-localfunctions!282 (merged). It tries to establish a cmake pattern for libraries: If you have a dedicated directory of tests in a dune module, all these tests typically should be linked against the library in that dune module. In order to inherit all compile flags and include directory, the test targets need to be linked against a dune target. This can be easily accomplished by linking all targets in a directory automatically against a common library, by using the link_libraries() command in the top of that directory. This is less error-prone than linking all targets individually.