MDEV-31349 test maria.maria-purge failed on 'aria_log.00000002 not found'

The bug was in the test case.
The problem was that maria_empty_logs.inc deleted aria log files before
the server was properly shutdown.
Fixed by waiting for pid file to disappear before starting to delete log
files.

Other things:
- Fixed that translog_purge_at_flush() will not stop deleting files even
  if one file could not be deleted.

mysql-test/include/maria_empty_logs.inc

@@ -11,6 +11,7 @@
 connection default;
 let $default_db=`select database()`;
 let $MYSQLD_DATADIR= `SELECT @@datadir`;
+let $pid_file=`select @@pid_file`;
 
 #it will used at end of test for wait_for_status_var.inc primitive
 #let $status_var= Threads_connected;
@@ -23,6 +24,7 @@ wait-maria_empty_logs.inc
 EOF
 
 --source include/mysqladmin_shutdown.inc
+--source include/wait_until_no_pidfile.inc
 
 --disable_warnings
 if (!$mel_keep_control_file)

mysql-test/include/wait_until_no_pidfile.inc

@@ -0,0 +1,30 @@
+# Include this script after a shutdown to wait until the pid file,
+# stored in $pid_file, has disappered.
+
+#--echo $pid_file
+
+--disable_result_log
+--disable_query_log
+# Wait one minute
+let $counter= 600;
+while ($counter)
+{
+--error 0,1
+--file_exists $pid_file
+  if (!$errno)
+  {
+    dec $counter;
+    --real_sleep 0.1
+  }
+  if ($errno)
+  {
+    let $counter= 0;
+  }
+}
+if (!$errno)
+{
+  --die Pid file "$pid_file" failed to disappear
+}
+
+--enable_query_log
+--enable_result_log