sim: testsuite: support parallel execution

Break up the dejagnu logic so that we can parallelize the testsuite.
This takes a page from gcc & gdb where each .exp is run in isolation
instead of in serial.

For most targets, this doesn't make much of a difference as they only
have a single .exp.  A few (like cris & frv) have multiple .exp though
and will see a bit of a speed up.

The real gain is when testing a multitarget build.  This way we can
run all the targets in parallel and cut the execution time a bit.
On my system, it goes from ~155sec to ~100sec.

We can gain further speedups by splitting up some of the larger .exp
files into smaller groups.  We'll do that in a followup though.