CMakeLists.txt: Compile the tests also with static tango library
In 76268474 (CMake on Linux: Only generate either the static or the dynamic library, 2020-08-28) we removed support for building the tests when having a static library (BUILD_SHARED_LIBS being OFF).
Turns out getting the tests compile with a static library is straightforward, so let's do it.
Close #1213 (closed)