make THEN step for visibility ignore irrelevant invisible elements

[dagroe]

Short:
when checking that COUNT elements are in/visible do not check the count of all found element but rather the count of those that are visible or invisible. This also allows to combine providing the COUNT with using the NOT option, e.g. Then 10 elements should not be visible.

Long:
I came across a scenario where one of multiple elements would be visible, e.g.

<BsCollapse @collapsed={{true}}>
  <div data-test-my-element>
    abc123
  </div>
</BsCollapse>
<BsCollapse @collapsed={{false}}>
  <div data-test-my-element>
    abc123
  </div>
</BsCollapse>

the following steps would fail:

Then I should see 1 MyElement
Then 1 MyElement should be visible

They would fail due to the check of collection count before getting to the check for actual visibility

"Then (?:(\\d+) )?$opinionatedElement should (not |NOT )?be visible"(countRaw, [collection/* , label, selector */], no) {
    assert(`Don't use NOT and number at the same time`, !(no && countRaw));

    let count =
      no       ? 0                      :
      countRaw ? parseInt(countRaw, 10) :
                 1;

    let m = `Element count`;
    expect(collection, m).to.have.length(count);

    collection.forEach((element, i) => {
      m = `Element #${i} (zero-indexed) visibility`;
      expect(isVisible(element), m).to.be.true;
    });
  }

So I suppose one can ignore the actual collection length but rather check the count based on each element's visibility.

[dagroe]

@lolmaus @simonihmig could you review this? Should I create an issue first?