[Development] Duplicated test data tags

[Edward Welbourne]

QTBUG-107185 revealed that QTest did not check for duplicated test data
tags, i.e. parameters to newRow() / addRow(); when I looked at the
implementation I found it also neglected to check for duplicated
addColumn names.  So [0] sets out to fix that.

It turns out that Qt itself has "quite a lot" of instances of duplicate
test data tags.  (I've only found one duplicate column, thankfully; but
there may be more.)  So, for now, the fix merely produces a warning; and
I do encourage all to watch out for "Duplicate data tag ... - please
rename." messages in test output (once [0] has landed).  Likewise for
similar with "column" in place of "tag".

[0] https://codereview.qt-project.org/c/qt/qtbase/+/436495

You can see from the branch it's the tip of, currently, some of the many
cases of duplication we had before it brought them to my attention.
There remain more even in qtbase and I haven't (for now, at least)
looked further afield.

The present draft of that change has #if-ery to turn duplication of
column or row names, in QTest data tables, at Qt 7, into a fatal error.
How realistic do folk think it is that client code authors will respond
to the warnings in time for that ?

I have, for now, included #if-ery complications that let you, when
building QtTestLib, define Q_TEST_REJECT_DUPLICATE_ROW to 0 or 1 in the
build system (and likewise for ..._COLUMN) to override the default
behaviour (0 means warning only even for Qt 7; 1 means fatal even before
Qt 7) but without any build infrastructure (yet) to set that.  Is this
adequate or overkill; does anyone have a better suggestion ?

	Eddy.