test: fix flaky test-fs-read-buffer-tostring-fail

The test is memory intensive and times out occasionally on Raspberry Pi
devices in CI. Successful test runs take about 90 seconds, but the
devices time out after 120 seconds. That's not a lot of headroom. So
let's skip the test on devices that have only modest amounts of memory.

Fixes: https://github.com/nodejs/node/issues/7042
PR-URL: https://github.com/nodejs/node/pull/7575


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>