Complex sentence: Testing of the MiG-15bis lasted 11 days revealing that overall, the aircraft was a reasonably good fighter, but lacked the technological sophistication of American aircraft, such as the F-86.
Split into simpler sentences: Testing of the MiG-15bis lasted 11 days. It revealed that the aircraft was a reasonably good fighter, but lacked the technological sophistication of American aircraft, such as the F-86.