seek test: remove trailing newline from filename

Looks like no one ever tried this?  fopen was returning EINVAL because
fgets was leaving a trailing newline at the end of the filename.
(Which is what fgets is documented to do.)

testcode/lib/seek.c

@@ -23,6 +23,7 @@ int main(int argc, char **argv)
         if (!x) {
             return(0);
         }
+        x[strcspn(x, "\r\n")] = 0;
         filename = x;
     }
     else {