Add unit test for realloc failure

Increases test coverage of realloc too!

Was:
             Hit   Total  Coverage
Lines:      1936   2574    75.2 %
Functions:   177    225    78.7 %
Branches:   1243   2360    52.7 %

Now:
Lines:      1945   2582    75.3 %
Functions:   177    225    78.7 %
Branches:   1247   2364    52.7 %

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Test zalloc() failure case.

This brings us to near total test coverage of core/malloc.c

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Add some (not that good) testing of mem_dump_allocs()

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>