deps: limit regress/regress-crbug-514081 v8 test

regress/regress-crbug-514081 allocates a 2G block of memory
and if there  are multiple variants running at the
same time this can lead to crashes, OOM kills or
the OS failing to allocate memory.  This patch
limits us to running a single variant of the test

Fixes: https://github.com/nodejs/node/issues/6340
PR-URL: https://github.com/nodejs/node/pull/6678


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>