Move expensive test to xt/, add malloc-canary preventing false-negatives

Move expensive test to xt/, add malloc-canary preventing false-negatives

This test was originally written to validate both Devel::GlobalDestruction::PP
and the M.A.D. cyclic reference handler (a4367b26). These days it makes little
sense to run on end-user installs, yet this bizarre test still uncovers weird
problems in the underlying Rube Goldberg machine.

So instead of outright deleting it - move it to xt/ and validate its execution
environment with what is essentially a guarded calloc()

Add a tight-memory travis config to make sure that OOM won't kill the wrong
thing

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