test: add Options classes tests and fix Arr::set return value bug

Add 27 wpunit tests covering get/set/delete/all/reset, defaults,
environment isolation, container singletons, and Plugin::option().

Fix Options::set() to capture the return value of Arr::set() which
returns a new array rather than mutating by reference.

Author: bordoni

src/WorkOS/Options/Options.php

@@ -57,9 +57,7 @@ public function get( string $key, $default = null ) {
 	 * @param mixed  $value Value to store.
 	 */
 	public function set( string $key, $value ): void {
-		$options = $this->all();
-		Arr::set( $options, $key, $value );
-		$this->options = $options;
+		$this->options = Arr::set( $this->all(), $key, $value );
 		$this->save();
 	}