Fix test to create and use post

If a WPGHS_Post was passed an ID that didn't exist in
the database, it would still be treated as new even if
it shouldn't have been. Once this bug was fixed, it
exposed this problem in the tests, which assumed we
could pretend that if a Post is passed an ID, it exists
in the DB. This resolves that inconsistency.

tests/unit/test-import.php

@@ -165,16 +165,13 @@ public function test_should_save_post_title_as_post_title() {
 	}
 
 	public function test_should_save_ID_as_post_id() {
+		$id = $this->factory->post->create();
 		$this->blob_content = 'Post content.';
-		$this->blob_meta    = array(
-			'ID' => 1
-		);
-		$this->post_meta    = array(
-			'_sha' => $this->sha,
-		);
+		$this->blob_meta    = array( 'ID' => $id );
+		$this->post_meta    = array( '_sha' => $this->sha );
 		$this->post_args    = array(
 			'post_content' => $this->blob_content,
-			'ID'           => 1,
+			'ID'           => $id,
 		);
 
 		$this->validate_meta();