Selector: Backport jQuery selection context logic to selector-native

This makes:
```js
$div.find("div > *")
```
no longer matching children of `$div`.

Also, leading combinators now work, e.g.:
```js
$div.find( "> *" );
```
returns children of `$div`.

As a result of that, a number of tests are no longer skipped in the
`selector-native` mode.

Also, rename `rcombinators` to `rleadingCombinator`.

Fixes gh-5185
Closes gh-5186
Ref gh-5085

1 files changed, 10 insertions, 0 deletions

diff --git a/src/selector/testContext.js b/src/selector/testContext.js
new file mode 100644
index 000000000..a54351e64
--- /dev/null
+++ b/src/selector/testContext.js
@@ -0,0 +1,10 @@
+/**
+ * Checks a node for validity as a jQuery selector context
+ * @param {Element|Object=} context
+ * @returns {Element|Object|Boolean} The input node if acceptable, otherwise a falsy value
+ */
+function testContext( context ) {
+	return context && typeof context.getElementsByTagName !== "undefined" && context;
+}
+
+export default testContext;