test: increase enoughTestMem to 1.75 Gb

PR-URL: https://github.com/nodejs/node/pull/16374
Fixes: https://github.com/nodejs/node/issues/16354


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>