crimson/test: shrink messenger-thrash max_message_len to 1MB

crimson/test: shrink messenger-thrash max_message_len to 1MB

The test keeps random_num payload buffers (each up to max_message_len)
resident and sends copies concurrently; at 4MB the peak working set
runs ~600MB and overflows the test's 256M --memory arena with bad_alloc.

--memory only binds when seastar's own allocator is compiled in
(RelWithDebInfo/Release); Debug/Sanitize define
SEASTAR_DEFAULT_ALLOCATOR and use the heap, so the limit never bites
there.

Shrink max_message_len to 1MB, cutting the peak below 160MB so it fits
the 256M arena instead of enlarging it.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>