sporadic failures due to full disk.

Fix by truncating general_log before altering it.
(if running the full main-test, it can be big).

mysql-test/r/partition.result:
  Bug#38120: main.partition fails sporadically
  
  updated result file
mysql-test/t/partition.test:
  Bug#38120: main.partition fails sporadically
  
  Truncating general_log before altering it.
  (if running the full main-test, it can be big
  when altering it).