oe-build-perf-test: suppress logger output when tests are being run

oe-build-perf-test: suppress logger output when tests are being run

Prevent logger from writing to stderr when the tests are being run by
the TestRunner. During this time the logger output is only written to
the log file. This way the console output from the script is cleaner and
not mixed with possible logger records.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>