Invert test stream capture logic for debugging.

Previously the default test behavior was to capture stdout and stderr.
Make the new default to not capture stdout and stderr then set
.testr.conf to always capture these streams when running tests under
testr.

The motiviation behind this change is that you will want these streams
to be captured when running under testr, but when not running under
testr you may not want to capture them. An example of this would be
running `python -m testtools.run test_name` with a change to test_name
to invoke the python debugger (capturing stdout and stderr interferes
with normal debugger functionality).

Also, only invoke the test timeout by default when running under testr.
This is done for the same reason as above. When running a test under the
debugger the timeout interferes with debugging.

Change-Id: I42cbbdadb2f221ec439e92a6800d14e8436bb77b
Merge "Invert test stream capture logic for debugging."