- disable the test suffering from Bug#41098 Query Cache returns wrong result with
                                            concurrent insert
- additional improvements

Details:
- Move the test for the historic bug
  Bug 28249 Query Cache returns wrong result with concurrent insert / certain lock
  into its own testscript query_cache_28249.test.
- query_cache.test:
  - replace error numbers with error names
  - remove trailing spaces, replace tabs with spaces
  - reset of @@global.log_bin_trust_function_creators to its original value
    at the end of the test
- query_cache_28249.test:
  - replace sleep 5 with poll routine
  - avoid random differences caused by concurrent_inserts effects
  - improved comments and formatting