Add README.md for tests

ballerina/tests/README.md

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+# Ballerina AWS Redshift Connector Test Module
+
+This test module is written to test the functionality of the Ballerina AWS Redshift Connector.
+
+## Docker Image
+For testing purposes, this [docker-pgredshift](https://github.com/HearthSim/docker-pgredshift/pkgs/container/docker-pgredshift) Docker image is used. It emulates AWS Redshift but with limited features, and it does not support SSL.
+
+## Emulated AWS Redshift Features
+The docker-pgredshift image provides a simulated environment resembling AWS Redshift. However, it is essential to be aware of the limited features and the absence of SSL support in this emulation.
+
+## Connection Details
+To establish a connection with the emulated AWS Redshift, the Ballerina AWS Redshift Connector uses the [Ballerina PostgreSQL Driver](https://github.com/ballerina-platform/module-ballerinax-postgresql.driver/) because the docker-pgredshift image is based on PostgreSQL and does not connect with the Redshift driver.
+
+## Usage
+Follow these steps to manually run the test module:
+
+1. Pull the docker-pgredshift image from [here](https://github.com/HearthSim/docker-pgredshift/pkgs/container/docker-pgredshift).
+2. Set up the docker-pgredshift container to emulate AWS Redshift.
+3. Use the Ballerina AWS Redshift Connector in your Ballerina programs to interact with the emulated AWS Redshift instance.
+
+## Reference Links
+- [docker-pgredshift Image](https://github.com/HearthSim/docker-pgredshift/pkgs/container/docker-pgredshift)
+- [Ballerina PostgreSQL Driver](https://github.com/ballerina-platform/module-ballerinax-postgresql.driver/)