MINOR: pools: switch the fail-alloc test to runtime only

MINOR: pools: switch the fail-alloc test to runtime only

The fail-alloc test used to be enabled/disabled at build time using
the DEBUG_FAIL_ALLOC macro, but it happens that the cost of the test
is quite cheap and that it can be enabled as one of the pool_debugging
options.

This patch thus introduces the first POOL_DBG_FAIL_ALLOC option, whose
default value depends on DEBUG_FAIL_ALLOC. The mem_should_fail() function
is now always built, but it was made static since it's never used outside.