test: fix Buffer OOM error message

Now that Buffers instantiate the Uint8Array in JS the error message has
changed in case the allocation fails due to OOM. Tests have been updated
to match.

PR-URL: https://github.com/nodejs/node/pull/2915


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>