swift234

build-script: allow combining '-t' and '-T' with other options again

This fixes a regression from PR #2213.

We needed a way to disable testing in a preset, even though previous
flags already requested tests.  So I added an argument to the '--test'
and '--validation-test' flags.  The argparse module treats '-T' and
'--validation-test' the same, so it thinks that in -RTi the 'i' is an
argument to '-T'.