C++ tests: Fix link errors on MSVC (regression from 2020-05-31). * modules/stdio-c++-tests (Makefile.am): Link test-stdio-c++ with $(LIB_GETRANDOM), needed for the rpl_tmpfile symbol. * modules/stdlib-c++-tests (Makefile.am): Link test-stdlib-c++ with $(LIB_GETRANDOM), needed for the mkdtemp and mkstemp symbols.