Fix failing test innodb.percona_changed_page_bmp_requests_debug

CURRENT_TEST: innodb.percona_changed_page_bmp_requests_debug --- /home/shurik/workspace/percona-server/8.0/mysql-test/suite/innodb/r/percona_changed_page_bmp_requests_debug.result  2020-06-10 21:14:36.877583707 +0300 +++ /home/shurik/workspace/percona-server/8.0/cmake-build-debug/mysql-test/var/log/percona_changed_page_bmp_requests_debug.reject       2020-09-02 11:52:12.899220702 +0300 @@ -35,38 +35,40 @@ SET @@GLOBAL.innodb_track_redo_log_now=TRUE;   # Listing .xdb files after the flush. -# The output must show 6 .xdb files: ib_modified_log_1_0.xdb to ib_modified_log_6_nnn.xdb. +# The output must show 6 .xdb files: ib_modified_log_1_0.xdb to ib_modified_log_7_nnn.xdb. ib_modified_log_1_<nnn>.xdb ib_modified_log_2_<nnn>.xdb ib_modified_log_3_<nnn>.xdb ib_modified_log_4_<nnn>.xdb ib_modified_log_5_<nnn>.xdb ib_modified_log_6_<nnn>.xdb +ib_modified_log_7_<nnn>.xdb +ib_modified_log_8_<nnn>.xdb   # Purging changed page bitmaps (tracking enabled). # All but the last one bitmap file must be deleted PURGE CHANGED_PAGE_BITMAPS BEFORE PREVIOUS_LSN;   # Listing .xdb files after the purge (tracking enabled). -# The output must show only one .xdb file - ib_modified_log_6_nnn.xdb. -ib_modified_log_6_<nnn>.xdb +# The output must show only one .xdb file - ib_modified_log_7_nnn.xdb. +ib_modified_log_8_<nnn>.xdb