Move test_auth_methods-t from legacy-g2 to mysql84-g2 only

Test requires MySQL 8.0+ (detects 5.7 and skips, but crashes with
'terminate called without an active exception' during noise thread
cleanup). The crash-on-skip is a separate test bug.

test/tap/groups/groups.json

@@ -217,7 +217,7 @@
   "stmt_explain-t" : [ "legacy-g2","mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
   "test_PROXY_Protocol-t" : [ "legacy-g3","mysql84-g3","mysql-auto_increment_delay_multiplex=0-g3","mysql-multiplexing=false-g3","mysql-query_digests=0-g3","mysql-query_digests_keep_comment=1-g3" ],
   "test_admin_stats-t" : [ "legacy-g2","mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
-  "test_auth_methods-t" : [ "legacy-g2","mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
+  "test_auth_methods-t" : [ "mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
   "test_auto_increment_delay_multiplex-t" : [ "legacy-g2","mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
   "test_backend_conn_ping-t" : [ "legacy-g2","mysql84-g2","mysql-auto_increment_delay_multiplex=0-g2","mysql-multiplexing=false-g2","mysql-query_digests=0-g2","mysql-query_digests_keep_comment=1-g2" ],
   "test_binlog_fast_forward-t" : [ "legacy-binlog-g1" ],