tapset: Add the macro @choose_defined and use it

tapset: Add the macro @choose_defined and use it

This returns the first parameter if it's @defined, otherwise it returns
the second parameter, replacing the common @defined(x)?x:y pattern.

I only updated the tapsets to use this, as I feel that's where we should
show off good code practice.  There are opportunities in the testsuite
that could use it too, but it's not necessary, and probably better to
leave existing tests undisturbed when possible.