PCT-638: QAN for slow log fails if slow_query_log_file is relative path

To make this testable I think we need to do:
```go
dataDir := m.GetGlobalVarString("datadir")
slowLog := m.AbsDataFile(dataDir, m. GetGlobalVarString("slow_query_log_file"))

func AbsDataFile(dataDir, fileName string) string {
   if !path.IsAbs(fileName) {
      fileName = path.Join(dataDir, fileName)
   }
   return fileName
}
```

Then just test `AbsDataFile` by passing in fake values, e.g. `("/dir", "slow.log")` expect `"/dir/slow.log"`.