provide test coverage for email form controls

[stopfstedt]

the tests/behat/groups.feature scenario used to have some of these, but they weren't working, and due to time constraints i had to yank them.

you can find them here:

moodle-block_quickmail/tests/behat/groups.feature

Lines 42 to 56 in 8e7f537

	When I click on "All Users" "text" in the "#groups" "css_element"
	Then "//select[@id='from_users']/option[text()='Student 1 (g1)' and @selected='selected']" "xpath_element" should exist
	And "//select[@id='from_users']/option[text()='Student 2 (g2)' and @selected='selected']" "xpath_element" should exist
	When I click on "All Groups" "text" in the "#groups" "css_element"
	Then "//select[@id='from_users']/option[text()='Student 1 (g1)' and @selected='selected']" "xpath_element" should exist
	And "//select[@id='from_users']/option[text()='Student 2 (g2)' and @selected='selected']" "xpath_element" should exist
	When I click on "g2" "text" in the "#groups" "css_element"
	Then "//select[@id='from_users']/option[text()='Student 1 (g1)' and @selected='selected']" "xpath_element" should not exist
	And "//select[@id='from_users']/option[text()='Student 2 (g2)' and @selected='selected']" "xpath_element" should exist
	When I click on "g1" "text" in the "#groups" "css_element"
	Then "//select[@id='from_users']/option[text()='Student 1 (g1)' and @selected='selected']" "xpath_element" should exist
	And "//select[@id='from_users']/option[text()='Student 2 (g2)' and @selected='selected']" "xpath_element" should not exist

so bring those back!

might need to roll some custom step definitions to make this happen - for example, i couldn't make selected-state checks happen on the options in the user selector using the built-in, xpath-powered steps.

but wait, there's more - cover the other interactions - user (un-)assignment, roles filter, etc - with tests as well.