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The C code produces correct values according to the tz database.
If TZ=Europe/Moscow then
tzname={"MSK", "MSD"} at 2010年07月01日 and
tzname={"MSK", "MSK"} at 2015年07月01日. Notice the difference!
The code calls C mktime() with corresponding time tuples
and checks that *tzname* is equal to the expected values. That's all.
If C code is incomprehensible; here's its Python analog:
 >>> import os
 >>> os.environ['TZ'] = 'Europe/Moscow'
 >>> import time
 >>> time.tzset()
 >>> time.mktime((2010,7,1,0,0,0,-1,-1,-1))
 1277928000.0
 >>> time.tzname #XXX expected ('MSK', 'MSD')
 ('MSK', 'MSK')
 >>> time.mktime((2015,7,1,0,0,0,-1,-1,-1))
 1435698000.0
 >>> time.tzname
 ('MSK', 'MSK')
C tzname changes on my machine after the corresponding C mktime() calls 
but Python time.tzname does not change after the time.mktime() calls.