Workaround 'uninitialized memory use' code analyzer false warning (tests)

Workaround 'uninitialized memory use' code analyzer false warning (tests)

* tests/test_atomic_include.template [AO_HAVE_storeXX && LINT2
&& AO_PREFER_GENERALIZED] (test_atomicXX): Initialize x local variable
(before AO_storeXX which can be defined using CAS); update comment.
* tests/test_atomic_include.template [AO_HAVE_short_storeXX && LINT2
&& AO_PREFER_GENERALIZED] (test_atomicXX): Initialize s local variable.
* tests/test_atomic_include.template [AO_HAVE_char_storeXX && LINT2
&& AO_PREFER_GENERALIZED] (test_atomicXX): Initialize b local variable.
* tests/test_atomic_include.template [AO_HAVE_int_storeXX && LINT2
&& AO_PREFER_GENERALIZED] (test_atomicXX): Initialize zz local variable.