fix: the number of concurrent job with --parallel option in mocha:cli:run:helpers

[sujin-park]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

When users run mocha with --parallel option wihtout --jobs option, undefined was passed to the parallelRun method.

I changed options.jobs to the number of CPU cores less 1 when options.jobs was false.

AS-IS

TO-BE

Alternate Designs

in lib/cli/run-helpers.js from line 179

AS-iS

options.jobs

TO-BE

options.jobs ? options.jobs : workerpool.cpus - 1

Why should this be in core?

It's a confirmed bug as described in #4415.

Benefits

Users can know exact current concurrent job count instead NaN.

Possible Drawbacks

N/A

Applicable issues

#4415

[coveralls]

Coverage increased (+0.3%) to 94.078% when pulling e0ae3a5 on sujin-park:sujin-park/fix-parallel into 4d7a171 on mochajs:master.

[boneskull]

Thanks! Just have a small suggestion to simplify things

[boneskull]

I would just remove the job count. It's printed in the debug logs elsewhere:

mocha/lib/nodejs/buffered-worker-pool.js

Lines 72 to 76 in e677219

	debug(
	'run(): starting worker pool of max size %d, using node args: %s',
	maxWorkers,
	process.execArgv.join(' ')
	);

maybe just say executing %d test file(s) in parallel mode

[sujin-park]

I fixed your feedback. I removed options.jobs params and added text in parallel mode.
Thank you for your comment.

[boneskull]

thanks!