Allow output.txt contents to be accessible from post_check function

[krishnan-gopal]

The simulation log output.txt file is empty (i.e not yet flushed) at the time when post_check is invoked for that simulation. The function needs to read the contents of this file in order to decide on PASS/FAIL in certain cases.
I like that fact that the post_check output itself goes to output.txt. But we still need a way to access the simulation transcript.
Maybe the post_check function can access the transcript<> file rather than output.txt ?

[kraigher]

The post_check function now accepts an output argument which is filled in with the full output string (The contents of output.txt upto calling post_check)

You can read about it here once Travis updates our website in a few minutes: