Accessing form elements work for both exact caption and caption-starts-with

With MGNLTEST-234 (Form#byFieldCaption can find String.startsWith) we have changed the API to access form fields in a way, that it does find a field not by the exact caption, but with a "caption starts with".

This change is still valuable - it will decrease the number of changes in tests in the context of re-adding the site, etc.  But the change also has broken at least one test (maybe more, but we cannot say this now, since dx-core@mster still is broken).

With this ticket we should ensure that the Form API allows both

• finding a form field with the exact caption
• finding a form field with caption-starts-with-string