fix UnitSwitchTests
fix SwitchTests
fix test_nalanda_input_exercise again
Merge pull request #2564 from jamalex/unit_setting_fixes

Allow unit to be changed even when original value was outside range.
set the default unit to the first entry in kalite.playlist.UNITS
set the unit of test_exam_mode_shut_out to 101
also set the unit first for test_set_current_unit_first
when calculating store transactions logs: ignore testlogs where students haven't answered a test
don't catch all exceptions when turning on/off the exam mode
Merge branch 'nalanda-rct3' of github.com:learningequality/ka-lite into fix_exercise_points