Update by.js: Add forgotten RelativeBy in check options

[AdamPDotty]

Description

The change expands a check for type By tp type RelativeBy to have later work, too.

Motivation and Context

I assume at some time RelativeBy was introduced. Several code places have been adjusted to accomodate the new type.
The changed line was forgotten und broke the function until.elementLocated() (and probably others) when used in connection with locateWith().
The function elementLocated() calls - like other functions, too - By.checkedLocator() which unnecessarily throws an error for parameters of type RelativeBy as produced by locateWith(). The function calls after the check have already been adjusted to work with RelativeBy.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.