runtc: unbreak the protocol decoder output match logic (type vs instance)

runtc: unbreak the protocol decoder output match logic (type vs instance)

Recent libsigrokdecode adjustment changed decoder output. The ID of a
protocol decoder instance need no longer match the decoder type's name
even for simple setups.

Lookup the selected decoder's instance ID during construction of the
decoder stack. Such that the selected decoder's data can get identified
in the output of the stack which is received by runtc(1).

Note that this implementation passes the decoder type's name to the
runtc(1) output file, so that it keeps matching the expected .output
files when pdtest(1) compares those two streams. This approach avoids
the necessity to adjust test.conf or .output files. It will no longer
be appropriate when more complex test configurations shall be supported
which involve multiple instances of the same decoder type or multiple
stacks of decoders.