Move all tests out of the csu subdirectory

Stack-protection on .o files in csu/ must be suppressed for the sake of
library startup code.  This also suppresses stack-protection in tests
(which are also covered by CFLAGS-.o), though this is neither necessary
nor desirable.

So impose the rule that .o files in csu/ are necessarily C startup code,
and move the few tests in there into misc/ instead.