make_and_run_test_nodes: avoid undefined behaviour

make_and_run_test_nodes: avoid undefined behaviour

In code that looks like n[i] = v(&i), where v increments i, C leaves it
undefined whether the old or new value of i is used to locate n[i].
As it happens, gcc used the pre-increment value of i, but MSVC
used the post-increment value.

Fix this by inserting a sequence point to disambiguate the intended order.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69924
Reviewed-by: Chengwei Yang <chengwei.yang@intel.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
[wrote commit message, fixed whitespace -smcv]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>