build-sys: added a 'test_Itemtables' to testing scheme

This change sets the stage for exploiting the recently
added ITEMTABLE_DEBUG #define. All tests are performed
in a single module (after trying 6 separate programs).

The chances of each test detecting errors is extremely
remote (at least while I'm maintaining these modules).
However, this single program approach has one flaw and
it relates to the response whenever an error is found.

Each of those six new API modules calls Exit() if they
detect an error. Otherwise, incorrect results would be
produced at the least or an abend encountered at most.

This means that multiple 'make check' invocations will
be needed if more than 1 module actually was in error.

All in all, it is a small price for a large assurance.

Reference(s):
https://www.freelists.org/post/procps/keep-on-patchin,7

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/.gitignore

@@ -1,3 +1,4 @@
+test_Itemtables
 test_namespace
 test_pids
 test_sysinfo