suite-add-option: failing test for duplicate, conflicting options

We shouldn't add an option to a suite if it already has that option, in
the code definition of the suite. In other words, options defined in the
code should have a higher precedence than options passed in at the
command line. This affects things like colouring output and choosing the
output format.
suite-add-option: pass test for not adding duplicate options

To avoid adding a duplicate option to a suite's options, check the
existing options for an option with the same key as the guven option.
Merge branch 'devel'
Merge branch 'master' into guile-2