test-char: fix AddressSanitizer failure

The CharSocketServerTestConfig and CharSocketClientTestConfig
objects escape after they are passed to g_test_add_data_func,
but they cease existing after the scope that defines them is
closed.  Make them static to fix this issue.

Fixes: e7b6ba41
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>