driver: add test for cy.within callback context, remove unnecessary ensureDom check

packages/driver/src/cy/commands/querying.coffee

@@ -418,7 +418,7 @@ module.exports = (Commands, Cypress, cy, state, config) ->
 
   Commands.addAll({ prevSubject: "dom"}, {
     within: (subject, options, fn) ->
-      cy.ensureDom(subject)
+      ctx = @
 
       if _.isUndefined(fn)
         fn = options
@@ -447,7 +447,7 @@ module.exports = (Commands, Cypress, cy, state, config) ->
       prevWithinSubject = cy.state("withinSubject")
       cy.state("withinSubject", subject)
 
-      fn.call cy.state("runnable").ctx, subject
+      fn.call(ctx, subject)
 
       cleanup = ->
         cy.removeListener("command:start", setWithinSubject)

packages/driver/test/cypress/integration/commands/querying_spec.coffee

@@ -219,6 +219,12 @@ describe "src/cy/commands/querying", ->
         cy.focused().should("have.class", "focused")
 
   context "#within", ->
+    it "invokes callback function with runnable.ctx", ->
+      ctx = @
+
+      cy.get("div:first").within ->
+        expect(ctx is @).to.be.true
+
     it "scopes additional GET finders to the subject", ->
       input = cy.$$("#by-name input:first")