Summary:
- Install unixodbc and postgres dlls & config for unixodbc
- Add `tests/test_odbc_connection.bats.in` test for ODBC testing
- Change `cmake/bats-tests.cmake` to add that file to testing if isql
  command is present. isql means that unixodbc was installed.
- Update testing instructions in `README.md` and `doc/intro.rst`.

Note the comments in the test script about various isssues.