MDL-76857 filter_displayh5p: Improve behat tests

Although the screenshots in the failures for some of the scenarios in
filter/displayh5p/tests/behat/h5p_filter.feature, like "Render a local
H5P file as teacher" were displaying the expected result, there were
some javascript errors (probably due to behat is quicker and the
iframes were not always ready).
I've added one extra step before accessing the iframe to give more
time to the H5P player to load and confirm the page is displayed
properly.

filter/displayh5p/tests/behat/h5p_filter.feature

@@ -69,6 +69,7 @@ Feature: Render H5P content using filters
     And I click on "Insert H5P" "button" in the "Insert H5P" "dialogue"
     And I wait until the page is ready
     When I click on "Save and display" "button"
+    And I should see "PageName1" in the "page-header" "region"
 #   Switch to iframe created by filter
     And I switch to "h5p-iframe" class iframe
 #   Switch to iframe created by embed.php page
@@ -104,6 +105,7 @@ Feature: Render H5P content using filters
     And I click on "Insert H5P" "button" in the "Insert H5P" "dialogue"
     And I wait until the page is ready
     When I click on "Save and display" "button"
+    And I should see "PageName1" in the "page-header" "region"
 #   Switch to iframe created by filter
     And I switch to "h5p-iframe" class iframe
     Then I should see "Note that the libraries may exist in the file you uploaded, but you're not allowed to upload new libraries."
@@ -129,6 +131,7 @@ Feature: Render H5P content using filters
     And I click on "Insert H5P" "button" in the "Insert H5P" "dialogue"
     And I wait until the page is ready
     And I click on "Save and display" "button"
+    And I should see "PageName1" in the "page-header" "region"
 #   Switch to iframe created by filter
     And I switch to "h5p-iframe" class iframe
 #   Libraries don't exist, so an error should be displayed.
@@ -152,6 +155,7 @@ Feature: Render H5P content using filters
     And I click on "Insert H5P" "button" in the "Insert H5P" "dialogue"
     And I wait until the page is ready
     And I click on "Save and display" "button"
+    And I should see "PageName2" in the "page-header" "region"
 #   Switch to iframe created by filter
     And I switch to "h5p-iframe" class iframe
 #   Switch to iframe created by embed.php page