The test started failing on the same day patch for BUG 49978 was
pushed. BUG 49978 changed part of the replication testing
infrastructure in mysql-test-run. This caused the test to fail
sporadically with result differences on relay log file
names. When the test fails the relay-log filenames are shifted by
one, eg:

-show relaylog events in 'slave-relay-bin.000002' from <binlog_start>;
+show relaylog events in 'slave-relay-bin.000003' from <binlog_start>;

The problem was caused by a bad cleanup when using the include
files:

  - include/setup_fake_relay_log.inc
  - include/cleanup_fake_relay_log.inc

Which would leave a spurious relay-log file around (not listed in
slave-relay-bin.index), causing the server to shift the name of
the relay logs by one, even if cleaning up with RESET SLAVE.

We fix this by removing the relay-log file when it is not needed
anymore, ie at setup time and after recreating the fake relay-log
index.

Additionally, to make the affected test more resilient, we
deployed a call to rpl_reset.inc (which resets both master and
slave, including log files) before actually running the test
case.

Finally, appart from the reported bug, we also fix: (a) an
unrelated issue with the failing test itself - in some cases, the
test was not setting the log file name to use when it should; 
(b) one typo.

mysql-test/extra/rpl_tests/rpl_show_relaylog_events.inc:
  Added call to rpl_reset.inc.
  Deployed missing instructions to get the binlog file name 
  before including show_relaylog/binlog_events.inc