Add test target.
New in 3.1:
* The generated C code is now in ANSI-C by default. If you want to support
pre-ANSI-C compilers, you need to provide the option --language=C on the
command line or %language=C in the source file.
* The 'len' parameter of the hash function and of the lookup function is now
of type 'size_t' instead of 'unsigned int'. This makes it safe to call these
functions with strings of length > 4 GB, on 64-bit machines.
* Added option --constants-prefix.
* Added declaration %define constants-prefix.
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