Instructions on running tests (#588)

* Add testing instructions

* Update link

docs/docs/developers/testing.md

@@ -0,0 +1,46 @@
+# Running tests locally
+
+## Prerequisites
+- Set `CORSO_PASSWORD` environment variable
+    ```bash
+    export CORSO_PASSWORD=<some password>
+    ```
+- Set AWS credential (needed for tests that use S3) environment variables
+    ```bash
+    export AWS_ACCESS_KEY_ID="...."
+    export AWS_SECRET_ACCESS_KEY="..."
+    export AWS_SESSION_TOKEN="..."
+    ```
+
+- Create a config file with the S3 bucket used for testing
+    ```toml
+    bucket = '<bucket name>'
+    ```
+- Set `CORSO_TEST_CONFIG_FILE` to use the test config file
+    ```bash
+    export CORSO_TEST_CONFIG_FILE=~/.corso_test.toml
+    ```
+- Set M365 Credentials environment variables
+    ```bash
+    export TENANT_ID=<tenant>
+    export CLIENT_ID=<id>
+    export CLIENT_SECRET=<secret>
+    ```
+
+## Running Tests
+Standard `go test ./...` will run unit tests
+
+Integration style tests are configured to run only if enabled by setting the
+appropriate ENV variable.
+
+e.g. `CORSO_CI_TESTS=true go test ./...`
+
+The complete list of enviroment constants is [here](../../../src/internal/tester/integration_runners.go)
+
+## Advanced Options
+
+- Use `CORSO_M356_TEST_USER_ID` to override the M365 user tests are run against
+
+    ```bash
+    export CORSO_M356_TEST_USER_ID="..."
+    ```