Remove set_compression program.

Adapt test cases to use modify instead.
diff --git a/regress/modify.c b/regress/modify.c
index af72f2f..28e1ae1 100644
--- a/regress/modify.c
+++ b/regress/modify.c
@@ -536,6 +536,8 @@
         return ZIP_CM_STORE;
     else if (strcmp(arg, "deflate") ==0)
         return ZIP_CM_DEFLATE;
+    else if (strcmp(arg, "unknown") ==0)
+        return 99;
     return 0; /* TODO: error handling */
 }