autotests: Make test framework a static lib again

Static plugins being linked with shared libraries doesn't work smoothly.
POSITION_INDEPENDENT_CODE target property needs to be set for
every such plugin. However, there can be targets that we have no control
over, which need POSITION_INDEPENDENT_CODE to be set too. Asking such
projects to add this property and them refusing to do so would be
reasonable because kwin's entire static plugin pipeline is weird.

The test framework was made a shared library because kwin build
directory used to get really big (in 10s of gigabytes).

Due to the -fPIC issues, this change makes the test framework a static
lib again. Obviously, this brings back the big build directory problem.
But, it's not as terrible as it used to be. With this change, kwin build
directory is a couple of gigabytes in debug build, which is a lot but
not as bad as it used to be before.