- adapted TestUnmarshalConfig to ensure the fields are read in
- added a test to see that at least MaxOpenConns works:
  - this is only exposed through (*db).Stats() in go 1.11, so this test
    has a build tag
  - the other two configurables can't be read back, so we've got to
    trust that the mechanism works given the one instance that's tested..

Signed-off-by: Stephan Renatus <srenatus@chef.io>