Fix unstable log verification in test_autovacuum.

Fix unstable log verification in test_autovacuum.

The test in test_autovacuum was unstable because it called
log_contains() immediately after verifying autovacuum_count in
pg_stat_user_tables. This created a race condition where the
statistics could be updated before the autovacuum logs were fully
flushed to disk.

This commit replaces log_contains() with wait_for_log() to ensure the
test waits for the parallel vacuum messages to appear. Additionally,
remove the checks of the autovacuum count. Verifying the log messages
is sufficient to confirm parallel autovacuum behavior, as logging is
only enabled for the specific table under test.

Per report from buildfarm member flaviventris.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/525d0f48-93f7-493f-a988-f39b460a79bc@gmail.com