tests: fix test failure due to os.freemem() behaviour change in node v17.4.0

[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

(Most test runs for all PRs will fail when testing against node v17 until this is merged in.)

[apmmachine]

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• Start Time: 2022-01-20T10:07:01.340+0000

• Duration: 23 min 16 sec

• Commit: 1868d7e

Test stats 🧪

Test	Results
Failed	0
Passed	243027
Skipped	0
Total	243027

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