test: refactor test-child-process-env to use arrow functions

[sagirk]

In test/parallel/test-child-process-env.js, callbacks use
anonymous closure functions. It is safe to replace them with arrow
functions since these callbacks don't contain references to this,
super or arguments. This results in shorter functions.

Checklist

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

[gireeshpunathil]

CI: https://ci.nodejs.org/job/node-test-pull-request/18757/

[sagirk]

@thefourtheye The build failed, seemingly because of the last change.

How do I go about fixing it? Undoing the common.mustCall wrapper will fix it, but I want to understand if there's something else that we can do here.

[Trott]

Please remove the use of common.mustCall() as an exit event handler. Thanks!

[sagirk]

@Trott Done. Build passed. PTAL.

[gireeshpunathil]

CI: https://ci.nodejs.org/job/node-test-pull-request/18873/

[gireeshpunathil]

windows-fanned test unrelated:

03:38:14 not ok 284 parallel/test-trace-events-worker-metadata
03:38:14 ---
03:38:14 duration_ms: 8.340
03:38:14 severity: fail
03:38:14 exitcode: 1
03:38:14 stack: |-
03:38:14 assert.js:351
03:38:14 throw err;
03:38:14 ^
03:38:14 03:38:14 AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:
03:38:14 03:38:14 assert(traces.some((trace) =>
03:38:14 trace.cat === '__metadata' && trace.name === 'thread_name' &&
03:38:14 trace.args.name === 'WorkerThread 1'))
03:38:14 03:38:14 at fs.readFile.common.mustCall (/home/iojs/build/workspace/node-test-binary-arm/test/parallel/test-trace-events-worker-metadata.js:26:7)
03:38:14 at /home/iojs/build/workspace/node-test-binary-arm/test/common/index.js:346:15
03:38:14 at FSReqCallback.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:54:3)
03:38:14 ...

[gireeshpunathil]

so all is good, just waiting for @Trott to re-review and dismiss the change request.

requested changes applied

[Trott]

Resume Build CI: https://ci.nodejs.org/job/node-test-pull-request/18989/

[gireeshpunathil]

landed as 484ad3b