crt: tests: remove explicit calls to `__mingw_printf`

Tests `t_float` and `t_speed_powl` directly call `__mingw_printf`; according
to relevant commit messages, it was done to suppress some compiler warnings.

Instead of calling `__mingw_printf` directly, define `__MINGW_USE_ANSI_STDIO`
to non-zero and use mingw-w64's `printf`.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>