Add zlib regression test

A zlib regression test has been added to verify the correct behavior
of z_compress_level() and z_uncompress.  The test case simply takes
a 128k buffer, it compresses the buffer, it them uncompresses the
buffer, and finally it compares the buffers after the transform.
If the buffers match then everything is fine and no data was lost.
It performs this test for all 9 zlib compression levels.

include/splat-ctl.h

@@ -104,6 +104,7 @@ typedef struct splat_cmd {
 #define SPLAT_SUBSYSTEM_LIST		0x0c00
 #define SPLAT_SUBSYSTEM_GENERIC		0x0d00
 #define SPLAT_SUBSYSTEM_CRED		0x0e00
+#define SPLAT_SUBSYSTEM_ZLIB		0x0f00
 #define SPLAT_SUBSYSTEM_UNKNOWN		0xff00
 
 #endif /* _SPLAT_CTL_H */