crt: tests: update t_winmain_assert

It is unclear what is the purpose of t_winmain_assert; it could be a simple
test program to observe behavior of assert() in an application using `WinMain`
as the entry point.

While this test uses `WinMain` as its entry point, it does not actually create
any windows, which allows to run it just like any other test program.

Update test test to assert that `lpCmdLine` argument is non-NULL.

Add t_winmain_assert to testcases/Makefile.am.

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