test: add dgram.Socket.prototype.sendto's test

[hiroppy]

sendto: https://github.com/nodejs/node/blob/master/lib/dgram.js#L234

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test

[cjihrig]

Can you store this regular expression in a variable for reuse.

[hiroppy]

Updated.

[cjihrig]

No need for the RegExp() constructor.

[cjihrig]

If all the sendto() calls are supposed to throw before actually sending, and we're verifying the error messages, I don't think binding, listening, and closing the socket is necessary.

[Trott]

LGTM if CI is green.

I'd leave a nit suggesting being more specific with the identifier name than just errorMessage but I'm having a hard time coming up with something significantly better that isn't probably too long, so ¯\(ツ)/¯.

[targos]

CI: https://ci.nodejs.org/job/node-test-pull-request/5982/

Tip for the link reference you put in the commit message: You can press Y on your keyboard to get an absolute link to the current revision. That way the line will remain correct even after the file is changed on master.

[hiroppy]

@targos Thanks :) I modified this commit message.

[targos]

Landed in 320108c

[MylesBorins]

failures on v4.x, did not land

=== release test-dgram-sendto ===
Path: parallel/test-dgram-sendto
assert.js:361
    throw actual;
    ^

Error: send takes offset and length as args 2 and 3
    at Socket.sendto (dgram.js:237:11)
    at /Users/mborins/code/node/v4.x/test/parallel/test-dgram-sendto.js:11:10
    at _tryBlock (assert.js:320:5)
    at _throws (assert.js:339:12)
    at Function.assert.throws (assert.js:369:3)
    at Object.<anonymous> (/Users/mborins/code/node/v4.x/test/parallel/test-dgram-sendto.js:10:8)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)