make: fix confusing debug logging when deleting a variable

Be clear when deleting a variable is ignored and state the reason,
instead of only providing a keyword as a hint for insiders.

tests/make: demonstrate confusing debug log for read-only variables

The message 'delete %s' that occurs above the 'readOnly' looks as if the
variable would indeed be deleted. The wording in the 'readOnly' line is
unclear.

Add the unit tests