`Wait For Condition` should validate that condition contains `return`

[pekkaklarck]

Wait For Condition requires the condition to return the value to be evaluated. For example, I just used

Wait For Condition    document.title == 'Example'

and it didn't work. After reading the docs I changed it to this working variant

Wait For Condition    return document.title == 'Example'

The keyword should check that there is return in the condition. This change can cause problems if the return isn't mandatory, but AFAIK it it.