bpo-45548: Some test modules must be built as shared libs (GH-29268)

bpo-45548: Some test modules must be built as shared libs (GH-29268)

Some test cases don't work when test modules are static extensions.

Add dependency on Modules/config.c to trigger a rebuild whenever a
module build type is changed.

``makesetup`` puts shared extensions into ``Modules/`` directory. Create
symlinks from pybuilddir so the extensions can be imported.

Note: It is not possible to use the content of pybuilddir.txt as a build
target. Makefile evaluates target variables in the first pass. The
pybuilddir.txt file does not exist at that point.