Issue #3618298: Fix the test content setup so every term has a referencing node

Fixes the exposed widget of the filter: BEF checkboxes lost their checked
state after a reload, and a dependent filter value that fell outside the
current option set broke the whole view.

What changed

• valueForm() now builds on InOperator::valueForm() and only adds the
  reference-view specifics (dependency JS, option order restore, hiding an
  empty filter). The option list, single/multiple handling, the - Any -
  fallback and the operators that take no value are core's job again.
• Plugin settings moved out of valueForm() into buildOptionsForm(),
  where they belong; valueForm() builds the value widget only.
• The unconditional user input rewrite is replaced by alignExposedInput(),
  which aligns the submitted value with the options currently available.
  This has to happen while the form is built: core rejects an unknown value
  in FormValidator::performRequiredValidation(), and Views then aborts the
  build, so the view rendered an error and no results at all.
• computeEffectiveValue() was already used by both the AJAX cascade and the
  widget; the change is that it no longer renumbers the values it returns.
  Checkbox widgets look their raw input up by option key, so the renumbered
  list left every box unchecked. EntityReferenceFilterRebuildCommand drops
  the keys again when it serialises selected, because the JavaScript checks
  that payload with Array.isArray().

Tests

• New coverage for the exposed widgets that had none: BEF checkboxes, a
  multi-value select, and the pruning of a stale dependent value.
• Test fixture fixes: the reference field value was written under a wrong
  key, so no node actually referenced a term; the fixture now creates one
  node per term.
• better_exposed_filters added as a dev dependency and to the test base
  classes — the test view has a BEF-rendered display and cannot be saved
  without it.