tests: fix test failure due to os.freemem() behaviour change in node v18

[trentm]

Recent node v18 nightly builds changed the behaviour of os.freemem()
on Linux to report "MemAvailable" from /proc/meminfo rather than
"MemFree". This broke our tests.

The nightly "edge" tests started failing ~6-7 days ago with:

running test: cd . && node --unhandled-rejections=strict test/metrics/index.test.js > test_output/test-metrics-index.test.js.tap 2&>1
...
[2022-01-14T19:31:11.076Z] node_tests_1     | # system.memory.actual.free
[2022-01-14T19:31:11.076Z] node_tests_1     | ok 11 is present
[2022-01-14T19:31:11.076Z] node_tests_1     | ok 12 is a number
[2022-01-14T19:31:11.076Z] node_tests_1     | ok 13 is finite (was: 10925056000)
[2022-01-14T19:31:11.076Z] node_tests_1     | not ok 14 is larger than os.freemem() (value: 10925056000, free: 10925056000)
[2022-01-14T19:31:11.076Z] node_tests_1     |   ---
[2022-01-14T19:31:11.076Z] node_tests_1     |     operator: ok
[2022-01-14T19:31:11.076Z] node_tests_1     |     expected: true
[2022-01-14T19:31:11.076Z] node_tests_1     |     actual:   false
[2022-01-14T19:31:11.076Z] node_tests_1     |     at: Object.system.memory.actual.free (/app/test/metrics/index.test.js:79:13)
[2022-01-14T19:31:11.076Z] node_tests_1     |     stack: |-
[2022-01-14T19:31:11.076Z] node_tests_1     |       Error: is larger than os.freemem() (value: 10925056000, free: 10925056000)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Test.assert [as _assert] (/app/node_modules/tape/lib/test.js:311:54)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Test.bound [as _assert] (/app/node_modules/tape/lib/test.js:96:32)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Test.assert (/app/node_modules/tape/lib/test.js:430:10)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Test.bound [as ok] (/app/node_modules/tape/lib/test.js:96:32)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Object.system.memory.actual.free (/app/test/metrics/index.test.js:79:13)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at Object.sendMetricSet (/app/test/metrics/index.test.js:137:20)
[2022-01-14T19:31:11.077Z] node_tests_1     |           at /app/lib/metrics/reporter.js:4:1202
[2022-01-14T19:31:11.077Z] node_tests_1     |           at /app/node_modules/after-all-results/index.js:20:25
[2022-01-14T19:31:11.077Z] node_tests_1     |           at processTicksAndRejections (node:internal/process/task_queues:78:11)
[2022-01-14T19:31:11.077Z] node_tests_1     |   ...

The breakage started in node nightly build "v18.0.0-nightly20220107b6b6510187".
The culprit node change was nodejs/node#41398, which upgrades libuv, which brought in a change to os.freemem() on Linux: libuv/libuv#3351

This broke an assumption in our tests.

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-01-14T22:06:47.927+0000

• Duration: 21 min 8 sec

• Commit: 98e243d

Test stats 🧪

Test	Results
Failed	0
Passed	242323
Skipped	0
Total	242323

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run module tests for <modules> : Run TAV tests for one or more modules, where <modules> can be either a comma separated list of modules (e.g. memcached,redis) or the string literal ALL to test all modules

• run benchmark tests : Run the benchmark test only.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)