support TRACE_FAILONLY env variable

In order to get similar behavior to the way testr out of the box
works, only displaying failing results, all the user to define
TRACE_FAILONLY environment variable (any value will do). After this
point only failures, and things leaking out of stderr will end up on
the screen.

We also need to remove the suppression of the inline fail dumps for
this to have the impact that you'd expect.

Change-Id: I72911adc8a03450522429c52db661ec0a1bc4678
Merge "support TRACE_FAILONLY env variable"