test-atomic: Fix warnings for GCC4.9 and sparse

There's no reason for the local variable 'old_count' to be atomic. In fact, if
it is atomic it triggers a GCC4.9 warning (Wunused-value)
The global variables 'a' and 'paux' could be static, according to sparse.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>