Update e2e Documentation with env var to set sample yaml files (#487)

* Update e2e Documentation with env var to set sample yaml files

Signed-off-by: Francisco H <frherrer@redhat.com>

* Minor improvements

Signed-off-by: Marko Lukša <mluksa@redhat.com>

---------

Signed-off-by: Francisco H <frherrer@redhat.com>
Signed-off-by: Marko Lukša <mluksa@redhat.com>
Co-authored-by: Marko Lukša <mluksa@redhat.com>

tests/e2e/README.md

@@ -15,7 +15,8 @@ This end-to-end test suite utilizes Ginkgo, a testing framework known for its ex
     1. [Pre-requisites](#Pre-requisites)
     1. [How to Run the test](#How-to-Run-the-test)
     1. [Running the test locally](#Running-the-test-locally)
-    1. [Settings for end-to-end test execution](#Settings-for-end-to-end-test-execution)    
+    1. [Settings for end-to-end test execution](#Settings-for-end-to-end-test-execution)
+    1. [Customizing the test run](#Customizing-the-test-run)    
     1. [Get test definitions for the end-to-end test](#Get-test-definitions-for-the-end-to-end-test)
 1. [Contributing](#contributing)
 
@@ -215,6 +216,23 @@ The following environment variables define the behavior of the test run:
 * DEPLOYMENT_NAME=sail-operator - The name of the operator deployment.
 * EXPECTED_REGISTRY=`^docker\.io|^gcr\.io` - Which image registry should the operand images come from. Useful for downstream tests.
 
+### Customizing the test run
+
+The test run can be customized by setting the following environment variables:
+
+To change all the sample files used in the test, you can use the following environment variable:
+* SAMPLE_YAML_BASE_URL=https://raw.githubusercontent.com/orgName/repoName - The base URL where the sample YAML files are located. This is useful when you want to test the operator with different sample YAML files defined in another repository. Currently the e2e framework use the upstream Istio repository to get the sample YAML files, for example: `https://raw.githubusercontent.com/istio/istio/master/samples/helloworld/helloworld.yaml` for `hello-world` sample yaml file on `master` branch.
+
+Note: when setting this environment variable, make sure that the sample files are available in the same paths as in the Istio upstream repository. For more information check the [code](https://github.com/istio-ecosystem/sail-operator/blob/74f96ab4a56e3d41e2d31faede290116a68551e9/tests/e2e/util/common/e2e_utils.go#L271).
+Alternatively, set the following environment variables to change these file paths:
+* TCP_ECHO_DUAL_STACK_YAML_PATH
+* TCP_ECHO_IPV4_YAML_PATH
+* TCP_ECHO_IPV6_YAML_PATH
+* SLEEP_YAML_PATH
+* HELLOWORLD_YAML_PATH
+
+`TCP_ECHO_*` are used in the `dual-stack` test suite, `SLEEP_YAML_PATH` and `HELLOWORLD_YAML_PATH` are used in both `Multicluster` and default test run.
+
 ### Get test definitions for the end-to-end test
 
 The end-to-end test suite is defined in the `tests/e2e/operator` directory. If you want to check the test definition without running the test, you can use the following make target: