test-genrandom: ensure stdout is set to _O_BINARY on Windows

test-genrandom: ensure stdout is set to _O_BINARY on Windows

Commit a6ca8c62 (Set _O_BINARY as default fmode for both MinGW and MSVC)
removed the definition of _CRT_fmode from mingw.c. Before this commit,
since test-genrandom is linked against libgit.a, the MinGW process
initialization code would pick up that definition of _CRT_fmode, which was
initialized to _O_BINARY. After this commit, however, text mode is used
for std(in|out|err) because it is the default in absence of _CRT_fmode.
In order to fix that, we must use git-compat-util.h, which overrides
main() to set the mode to binary.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>