tests: fix test failure due to os.freemem() behaviour change in node v16.14.0

[trentm]

The update to libuv 1.43.0 changed the behaviour of os.freemem() on
Linux to report "MemAvailable" from /proc/meminfo rather than "MemFree".

Refs: #2530, #2540

This is the same thing as we have already done for node v18 and v17, respectively, in the two linked PRs. The recently released node v16.14.0 (https://nodejs.org/en/blog/release/v16.14.0/) updated to the libuv with this os.freemem() behaviour change:

• [af6f1d512d] - deps: upgrade to libuv 1.43.0 (Colin Ihrig) #41398

[apmmachine]

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• Start Time: 2022-02-10T18:29:30.707+0000

• Duration: 34 min 12 sec

Test stats 🧪

Test	Results
Failed	0
Passed	240407
Skipped	0
Total	240407

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[trentm]

The test failure on "main" that led to this:

https://apm-ci.elastic.co/blue/organizations/jenkins/apm-agent-nodejs%2Fapm-agent-nodejs-mbp/detail/main/39/pipeline

[2022-02-10T06:17:59.323Z] node_tests_1     | running test: cd . && node --unhandled-rejections=strict test/metrics/index.test.js > test_output/test-metrics-index.test.js.tap 2&>1
...
[2022-02-10T06:17:59.585Z] node_tests_1     | # system.memory.actual.free
[2022-02-10T06:17:59.585Z] node_tests_1     | ok 74 is present
[2022-02-10T06:17:59.585Z] node_tests_1     | ok 75 is a number
[2022-02-10T06:17:59.586Z] node_tests_1     | ok 76 is finite (was: 10904395776)
[2022-02-10T06:17:59.586Z] node_tests_1     | not ok 77 is larger than os.freemem() (value: 10904395776, free: 10904395776)
[2022-02-10T06:17:59.586Z] node_tests_1     |   ---
[2022-02-10T06:17:59.586Z] node_tests_1     |     operator: ok
[2022-02-10T06:17:59.586Z] node_tests_1     |     expected: true
[2022-02-10T06:17:59.586Z] node_tests_1     |     actual:   false
[2022-02-10T06:17:59.586Z] node_tests_1     |     at: Object.system.memory.actual.free (/app/test/metrics/index.test.js:85:13)
[2022-02-10T06:17:59.586Z] node_tests_1     |     stack: |-
[2022-02-10T06:17:59.586Z] node_tests_1     |       Error: is larger than os.freemem() (value: 10904395776,
[2022-02-10T06:17:59.586Z] node_tests_1     |                 free: 10904395776)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Test.assert [as _assert] (/app/node_modules/tape/lib/test.js:314:54)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Test.bound [as _assert] (/app/node_modules/tape/lib/test.js:99:32)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Test.assert (/app/node_modules/tape/lib/test.js:433:10)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Test.bound [as ok] (/app/node_modules/tape/lib/test.js:99:32)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Object.system.memory.actual.free (/app/test/metrics/index.test.js:85:13)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at Object.sendMetricSet (/app/test/metrics/index.test.js:144:20)
[2022-02-10T06:17:59.586Z] node_tests_1     |           at /app/lib/metrics/reporter.js:4:1202
[2022-02-10T06:17:59.586Z] node_tests_1     |           at /app/node_modules/after-all-results/index.js:20:25
[2022-02-10T06:17:59.586Z] node_tests_1     |           at processTicksAndRejections (node:internal/process/task_queues:78:11)
[2022-02-10T06:17:59.586Z] node_tests_1     |   ...
[2022-02-10T06:17:59.586Z] node_tests_1     | # system.process.memory.rss.bytes
...