Fix Windows test compilation: mkdtemp, mkdir, setenv, pipe compat

- cbm_mkdtemp: _mktemp + _mkdir on Windows
- cbm_setenv/cbm_unsetenv: _putenv_s on Windows
- cbm_pipe: _pipe on Windows
- Replace bare POSIX calls in 12 test files with compat wrappers
- All test files now include compat.h