lib: fix coverage reporting

Taking the source code of a function and running it in another
context does not play well with coverage instrumentation.
For now, do the simple thing and just write the source code as
a string literal.

Fixes: https://github.com/nodejs/node/issues/19912
Refs: https://github.com/nodejs/node/pull/19524

PR-URL: https://github.com/nodejs/node/pull/20035


Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>