MDL-43713 behat: improve multi-select support

This patch implements:

1) Normalization of options. Before the patch options
in a select were being returned as "op1 op2 op3" by selenium
and "op1 op2 op3" by goutte. With the patch, those lists
are always returned like "op1, op2, op3". If real commas are
needed when handling multiple selects they should
be escaped with backslash in feature files.

2) Support for selecting multiple options. Before the patch
only one option was selected and a new selection was cleaning the
previous one. With the patch it's possible to pass "op1, op2" in
these steps:
  - I fill the moodle form with (table)
  - I select "OPTION_STRING" from "SELECT_STRING"

3) Ability to match multiple options in this steps. Before the
patch matching of multiple was really random, now every every
passed option ("opt1, opt2") is individually verified. It applies
to these 2 steps:
  - the "ELEMENT" select box should contain "OPTIONS"
  - the "ELEMENT" select box should not contain "OPTIONS"

4) Two new steps able to verify if a form have some options selected or no:
  - the "ELEMENT" select box should contain "OPTIONS" selected
  - the "ELEMENT" select box should contain "OPTIONS" not selected

5) Change get_value from xpath search to Mink's getValue() that is immediate
(does not need form submission) and works for all browsers but Safari, that
fails because of the extra ->click() issued.

Note all the changes 1-4 only affect to multi-select fields. Single
selects should continue working 100% the same.

The change 5) causes Safari to fail. The problem has been traced down to
the extra ->click() present there. Anyway there are not test cases
requiring that "immediate" evaluation right now. Only the special feature
file attached verifies it.

The issue (MDL-43713) contains a feature testing all these changes.