fix building test programs with gnu make 3.82
Test programs may be named either .c or %-test.c. In some cases (ie. fft),
both files exist and make has to choose which rule to use. Up to gnu make
3.81, rules were applied in definition order, in this case favouring the
generic %.c rule from common.mak. gnu make 3.82 uses a shortest stem first
order, applying definition order for same length stems. In this case,
-test.c must come first in order for make to pick the proper source file.
Originally committed as revision 25918 to svn://svn.ffmpeg.org/ffmpeg/trunk