We weren't generating a stats file for tests that only had stats_range_fields perf tests

I fixed this in a plausible looking way, similar to compiler_stats_range/num fields.

testsuite/driver/testlib.py

@@ -1165,7 +1165,7 @@ def simple_run( name, way, prog, args ):
     my_rts_flags = rts_flags(way)
 
     stats_file = name + '.stats'
-    if len(opts.stats_num_fields) > 0:
+    if len(opts.stats_num_fields) + len(opts.stats_range_fields) > 0:
         args += ' +RTS -V0 -t' + stats_file + ' --machine-readable -RTS'
 
     if opts.no_stdin: