test: removed unnecessary Buffer import

no need to require('buffer') in test files, so removed from:

• test\disabled\test-sendfd.js
• test\internet\test-dgram-broadcast-multi-process.js
• test\pummel\test-https-no-reader.js

first ever contribution :-) thanks to #goodnessSquad for the support and vision!

Refs: https://github.com/nodejs/node/issues/13836

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows [commit guidelines]

Affected core subsystem(s)

test