Inconsistent shared library name prefix on MinGW and Cygwin between shared_library() and test cases

[na-trium-144]

The documentation of shared_library() says the default prefix of shared library name is omitted on MSVC, cyg on Cygwin, and lib otherwise.
However, it seems the unit test (run_project_tests.py, line 199) assumes that the name of shared library is foo.dll or foo-X.dll on Windows, including MinGW and Cygwin.
I found this while working on the test case common/230 external project on Msys2 in #13916, but I'm not sure if fixing this will affect other working unit tests, so instead I patched the specific test case configuration as for now.