Default to not capturing log output in tests

Viewing log output while tests are still running can be useful for
debugging, but log output was previously always captured.  This
change ensures that log capture is off by default, but can still
be enabled by setting OS_LOG_CAPTURE=1 in the shell environment.

testr invocation is unchanged and will continue to capture logs by
default.

Change-Id: I9d0fb648541280cacfebb47f67f608378ae66ef3
Merge "Default to not capturing log output in tests"