Factor out test execution into t/trace_*_common.py, which defines
a 'run' functions. All related t_trace_*py tests call this function.
Behaviour is partially encoded in the file name, which must be of the
form: t_trace_complex_<variant>_<mode>_<format>*.py, where '<variant>'
determines tracing options (default/params/structs), '<mode>' is the
compilation mode (--cc/--sc), and '<format>' determines the trace format
(vcd/fst/saif).
The part of the test name after '<format>' does not influence the test
directly and can be free form. If used, explicit 'verilator_flags2' is
passed to the 'run' function.
4b53f5f978