mtr uses group suffix, but some existing inc and test files use
server_id for expect files. This patch aims to fix that.

For spider:

With this change we will not have to maintain a separate version of
restart_mysqld.inc for spider, that duplicates code, just because
spider tests use different names for expect files, and shutdown_mysqld
requires magical names for them.

With this change spider tests will also be able to use other features
provided by restart_mysqld.inc without code duplication, like the
parameter $restart_parameters (see e.g. the testcase mdev_29904.test
in commit ef1161e5d4f).

Tests run after this change: default, spider, rocksdb, galera, using
the following command

mtr --parallel=auto --force --max-test-fail=0 --skip-core-file
mtr --suite spider,spider/*,spider/*/* \
    --skip-test="spider/oracle.*|.*/t\..*" --parallel=auto --big-test \
    --force --max-test-fail=0 --skip-core-file
mtr --suite galera --parallel=auto
mtr --suite rocksdb --parallel=auto