Add support for running integration tests with dockerized test databases.

tests/Readme.md

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+
+# Integration tests
+
+This folder contains integration tests intended to test jet generator, statements and query result mapping with a running database.
+
+## How to run tests?
+
+Before we can run tests, we need to set up and initialize test databases.
+To simplify the process there is a Makefile with a list of helper commands.
+```shell
+# We first need to checkout testdata from separate repository into git submodule,
+# then download docker image for each of the databases listed in docker-compose.yaml file, and 
+# finally run and initialize databases with downloaded test data.
+# Note that on the first run this command might take a couple of minutes.
+make setup
+
+# When databases are ready, we can generate sql builder and model types needed to compile the tests.
+make jet-gen
+```
+
+Then we can run the tests the usual way:
+```shell
+go test -v ./...
+```
+
+To removes test containers, volumes, and images:
+```shell
+make cleanup
+# If temp ./.docker folder is still not empty, use 'sudo rm -rf .docker' to remove it.
+```