test: refactor test-dgram-msgsize.js

[dguo]

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

Change var to const, and change assert.equal() to assert.strictEqual().

[mcollina]

CI: https://ci.nodejs.org/job/node-test-pull-request/5229/

[mcollina]

Failure unrelated. Merging.

[mcollina]

Impossible to merge, the same changes are part of #9926.

[Trott]

Sorry about giving you a duplicate task! I see that you got a few other PRs open from Code & Learn, so that's a relief. :-D

Anyway, this was my mistake, so I apologize!

[dguo]

Oh, it's no problem. Thanks for MCing the event! It was fun.