Updated test for range slider in time slider.

Instead of raw comparison against an HTML string to get style information,
now proper selenium function is used to get value of CSS property.

cms/djangoapps/contentstore/features/video.feature

@@ -86,4 +86,4 @@ Feature: CMS.Video Component
     And I set value "24" to the field "End Time"
     And I save changes
     And I click button "Play"
-    Then I see a range on slider with styles "left" set to 10 and "width" set to 10
+    Then I see a range on slider with styles "left" set to 70 px and "width" set to 70 px

cms/djangoapps/contentstore/features/video.py

@@ -203,14 +203,14 @@ def click_button(_step, button_type):
     world.css_click(BUTTONS[button])
 
 
-@step('I see a range on slider with styles "left" set to (.+) and "width" set to (.+)$')
+@step('I see a range on slider with styles "left" set to (.+) px and "width" set to (.+) px$')
 def see_a_range_slider_with_proper_range(_step, left, width):
     left = int(left.strip())
     width = int(width.strip())
 
     world.wait_for_visible(".slider-range")
-    slider_range = world.css_find(".slider-range")[0]
+    slider_range = world.browser.driver.find_element_by_css_selector(".slider-range")
 
-    assert "left: {}%;".format(left) in slider_range.outer_html
-    assert "width: {}%;".format(width) in slider_range.outer_html
+    assert int(round(float(slider_range.value_of_css_property("left")[:-2]))) == left
+    assert int(round(float(slider_range.value_of_css_property("width")[:-2]))) == width