tests: Silence false positive warning on generated test name

Running 'make check' on rawhide with gcc 8.0.1 fails:

tests/test-visitor-serialization.c: In function 'main':
tests/test-visitor-serialization.c:1127:34: error: '/primitives/' directive writing 12 bytes into a region of size between 1 and 128 [-Werror=format-overflow=]

The warning is a false positive (we have two buffers of size 128,
so yes, if we FULLY used the first buffer, then sprint'ing it into
the second will overflow the second).  But in practice, our first
buffer will not be longer than "/visitor/serialization/String",
so sizing it smaller is enough to let gcc see that we don't
overflow the second.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180323204341.1501664-1-eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>