test: refactor test-vm-sigint

[Trott]

• Use common.mustNotCall() to confirm SIGINT listeners are not being
  invoked.
• Improve assertion check on integer child argument.
• Add blank line per test writing guide.

Checklist

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

Affected core subsystem(s)

test vm

[Trott]

CI: https://ci.nodejs.org/job/node-test-pull-request/8819/

[cjihrig]

Only test failure looks unrelated.

[Trott]

Closed in 99dc9d0

[MylesBorins]

This does not land cleanly in LTS. Please feel free to manually backport by following the guide. Please also feel free to replace do-not-land if it is being backported

It is worth mentioning that after fixing the conflicts there were a bunch of errors on v6.x

=== release test-vm-sigint === Path: parallel/test-vm-sigint /Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js:23 assert.ok(Number.isInteger(listeners)); ^

ReferenceError: listeners is not defined
at Object. (/Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js:23:30)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)
at run (bootstrap_node.js:389:7)
at startup (bootstrap_node.js:149:9)
at bootstrap_node.js:502:3
/Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js:23
assert.ok(Number.isInteger(listeners));
^

ReferenceError: listeners is not defined
at Object. (/Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js:23:30)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.runMain (module.js:604:10)
at run (bootstrap_node.js:389:7)
at startup (bootstrap_node.js:149:9)
at bootstrap_node.js:502:3

assert.js:81
throw new assert.AssertionError({
^
AssertionError: 1 === 0
at ChildProcess.child.on.common.mustCall (/Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js:51:12)
at ChildProcess. (/Users/mborins/code/node/v6.x/test/common/index.js:464:15)
at emitTwo (events.js:106:13)
at ChildProcess.emit (events.js:191:7)
at maybeClose (internal/child_process.js:891:16)
at Socket. (internal/child_process.js:342:11)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at Pipe._handle.close [as _onclose] (net.js:497:12)
Command: out/Release/node /Users/mborins/code/node/v6.x/test/parallel/test-vm-sigint.js