Styles API: Fixed selectors for nested elements

[MaggieCabrera]

What?

This PR solves an issue that was uncovered while backporting the changes to the Elements API.

Why?

A failing test was found that is present in core but not on GB, this test shows that if a theme defines styles for a link inside the heading block, only the last selector of the list of headings will apply those styles (h6).

This PR is bringing the test that was failing on core to GB, since it definitely proved useful in this particular case.

How?

The code was assuming an array of selectors for the elements but then it was overriding it with a string, only saving the last one in the list. This PR fixes that, applying the link selector to all the headings.

Testing Instructions

On your theme (I used emptytheme) add this to theme.json:

	"styles": {
		"blocks": {
			"core/heading":{
				"elements": {
					"link": {
						"color": {
							"text": "red"
						}
					}
				}
			}
		}
	}

Post headings with links. Before this PR, only h6 will be red; after this PR, all of them show red links.

--

Can someone explain why we don't have the same tests in both core and GB? Or at least why the tests in core aren't running for GB commits. Having that core test would have caught this bug on the original PR, not in the backport one.

[cbravobernal]

It works as described. Nice work!
I don't know the reason for this test not being in Gutenberg and being in Core 😢