_auto_signal_file_names: fix counting channels per group.

Since the FLAC format has a hard limit of eight channels, we need to
use multiple signal files if a record contains more than eight
signals.

Commit d1d26ba was meant to do this automatically (when
Record.set_default or wfdb.wrsamp is used to generate the signal file
names), but this was never tested.

Author: Benjamin Moody

wfdb/io/_header.py

@@ -384,6 +384,7 @@ def _auto_signal_file_names(self):
                     num_groups += 1
                     channels_in_group = 0
             group_number.append(num_groups)
+            channels_in_group += 1
             prev_fmt = ch_fmt
             prev_spf = ch_spf