- moved the test into a separate file to check for presence of the test variable

mysql-test/r/subselect.result:
  Bug#37627: moved the test to a separate file
mysql-test/r/subselect_debug.result:
  Bug#37627: moved the test to a separate file
mysql-test/t/subselect.test:
  Bug#37627: moved the test to a separate file
mysql-test/t/subselect_debug.test:
  Bug#37627: moved the test to a separate file