testsuite: split out function to compare outputs exactly

testsuite: split out function to compare outputs exactly

Move functionality to compare the exact output to a separate function
and allocate one buffer per output/match pair. This will allow us to
extend this to allow other types of comparisons. Since now we are using
heap-allocated buffer, keep the buffer allocation to the caller, so we
don't have to allocate and free it on every invocation. It also avoids
the different comparison functions to have to deal with it.