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".

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) Fix get_value() logic to use Mink's getValue() instead of performing
xpath searches. That way no submission + re-edition is required to get
tests working.

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