keep hierarchy for skipped suites w/o a callback

[outsideris]

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

As my local test for #3184, describe.skip without a callback couldn't keep a hierarchy of suites.

For Instance.

var assert = require('assert');

describe("Some tests", function(){
  describe.skip("Some disabled test", function() {});
});

describe("Some other tests", function(){
  it("should still pass", function(){
    assert.equal(1,1);
  });
});

is show below output.

  Some other tests
    ✓ should still pass


  1 passing (5ms)

It is correct. But when omit a callback in describe.skip.

var assert = require('assert');

describe("Some tests", function(){
  describe.skip("Some disabled test");
});

describe("Some other tests", function(){
  it("should still pass", function(){
    assert.equal(1,1);
  });
});

A hierarchy of the suites is wrong like below.

  Some tests
    Some other tests
      ✓ should still pass


  1 passing (5ms)

So, this PR add suites.shift(); for a skipped suite w/o a callback.

Alternate Designs

I'm not sure.

Why should this be in core?

Some test can be skipped not intentionally.

Benefits

Skipped suites don't make side effect for other tests.

Possible Drawbacks

I'm not sure.

Applicable issues

#3184

I don't agree #2014 is related to this issue.

[coveralls]

Coverage increased (+0.05%) to 90.0% when pulling b844006 on outsideris:issue-3184 into ec8901a on mochajs:master.

[boneskull]

Hi, thanks. I think this looks OK, but please see a couple comments I have.

I'll probably add more coverage around this after it's merged.

[boneskull]

please simplify

} else if (!opts.fn && suite.pending) {

[boneskull]

spelling: "another"

[outsideris]

Thank you for the feedback.
I updated it and rebase onto master branch.

[boneskull]

@outsideris Thanks!