The reason for the bug above is unclear but
- Modify pfs_upgrade so that it's result is easier to analyze in case something fails
- Fix several minor weaknesses which could cause that a successing test (either an
  already existing or a to be developed one) fails because of imperfect cleanup,
  too slow disconnected sessions etc.
should either fix the bug or reduce it's probability or at least
make the analysis of failures easier.

mysql-test/suite/perfschema/include/upgrade_check.inc:
  New include file which contains redundant stuff taken from pfs_upgrade.test.
  Remove any file which might harm analysis of suspicious results.
mysql-test/suite/perfschema/r/query_cache.result:
  Updated results
mysql-test/suite/perfschema/r/selects.result:
  Updated results
mysql-test/suite/perfschema/t/bad_option_1.test:
  Add the missing remove_file at beginning and end of test.
mysql-test/suite/perfschema/t/bad_option_2.test:
  Add the missing remove_file at beginning and end of test.
mysql-test/suite/perfschema/t/global_read_lock.test:
  Add a wait routine which ensures that the disconnect is really completed when the test ends.
mysql-test/suite/perfschema/t/pfs_upgrade.test:
  - Move redundant actions to include/upgrade_check.inc
  - Add preemptive removal of files
mysql-test/suite/perfschema/t/privilege.test:
  Add a wait routine which ensures that the disconnect is really completed when the test ends.
mysql-test/suite/perfschema/t/query_cache.test:
  Add "flush status" so that counters are reset. (./mtr --repeat=2 perfschema.query_cache failed)
mysql-test/suite/perfschema/t/read_only.test:
  Add a wait routine which ensures that the disconnect is really completed when the test ends.
mysql-test/suite/perfschema/t/selects-master.opt:
  Needed for running with enabled event-scheduler
mysql-test/suite/perfschema/t/selects.test:
  - Correct the sub test for the EVENT scheduler
  - Replace "sleep" by wait_routine
  - Add premptive cleanups like "DROP ... IF EXISTS ..."