test: increase RAM requirement for intensive tests

test-fs-read-buffer-tostring-fail and test-fs-readfile-tostring-fail
have been timing out on Raspberry Pi 3 devices on the continuous
integration server. These devices have 1 Gb of RAM and the tests are
memory intensive. Previous checks for memory intensive tests used a 512
Mb cut-off, but that was probably instituted when we only had Pi 1
devices.

Consequently, this change increases the threshold for memory-intensive
tests to 1 Gb and adds that threshold to test-fs-readfile-tostring-fail.

PR-URL: https://github.com/nodejs/node/pull/7772


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>