2. Slice of fix for Bug#42003 tests missing the disconnect of connections <> default

- If missing: add "disconnect <session>"
   - If physical disconnect of non "default" sessions is not finished
     at test end: add routine which waits till this happened
+ additional improvements
  - remove superfluous files created by the test
  - replace error numbers by error names
  - remove trailing spaces, replace tabs by spaces
  - unify writing of bugs within comments
  - correct comments
  - minor changes of formatting
Fixed tests:
  backup
  check
  compress
  grant
  information_schema
  multi_update
  overflow
  packet
  query_cache_not_embedded
  sp-threads
  subselect
  synchronization
  timezone_grant

mysql-test/t/compress.test

@@ -6,6 +6,10 @@
 
 -- source include/have_compress.inc
 
+# Save the initial number of concurrent sessions
+--source include/count_sessions.inc
+
+
 connect (comp_con,localhost,root,,,,,COMPRESS);
 
 # Check compression turned on
@@ -16,3 +20,10 @@ SHOW STATUS LIKE 'Compression';
 
 # Check compression turned on
 SHOW STATUS LIKE 'Compression';
+
+connection default;
+disconnect comp_con;
+
+# Wait till all disconnects are completed
+--source include/wait_until_count_sessions.inc
+