process: add coverage tests for sourceMapFromDataUrl method

PR-URL: https://github.com/nodejs/node/pull/30319


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>