Update readme regarding --preserve-installers switch (#54)

* Update readme regarding --preserve-installers switch
* Add documentation on how to list and download installers with specific version

roles/oneagent/tests/README.md

@@ -26,9 +26,12 @@ local deployment are specified or any other platforms is used along with local o
 - venv 20.0+
 - 
 ## Running tests
+
+### Preparing test environment
+
 Upon downloading the collection
 
-```commandline
+```console
 # Install dependencies
 $ apt-get install -y python3-venv python3-pip sshpass
 
@@ -43,11 +46,38 @@ $ pip install -r roles/oneagent/tests/requirements.txt
 $ mkdir -p roles/oneagent/files && wget https://ca.dynatrace.com/dt-root.cert.pem -P roles/oneagent/files
 $ ansible-galaxy collection build . -vvv
 $ sudo bash -c "source venv/bin/activate && ansible-galaxy collection install -vvv dynatrace-oneagent*"
+```
+
+### Running tests locally and remotely
+
+Running tests with regular manner requires one of the following commands:
 
+```console
 # Run tests for any platform (except from Windows) on local machine
 $ sudo bash -c "source venv/bin/activate && pytest roles/oneagent/tests --linux_x86=localhost"
 
 # Run tests with regular installer on remote Windows machine
 $ sudo bash -c "source venv/bin/activate && pytest roles/oneagent/tests --user=<USER> --password=<password> \
 --tenant=https://abc123456.com --tenant_token=<TOKEN> --windows_x86=<IP>"
 ```
+
+There is also an option to run tests with placed-in installers using `--preserve-installers` switch. 
+In this mode, the test environment won't be fully cleaned. It requires that both installers differs in version and have 
+the naming schema from tenant, e.g. `Dynatrace-OneAgent-Linux-1.301.0.sh`, `Dynatrace-OneAgent-Linux-arm-1.302.0.sh`.
+Also, the installers certificate must be downloaded for successful run. </br>
+You can refer to [this documentation](https://docs.dynatrace.com/docs/shortlink/api-deployment-get-versions) on how to
+list available installers.
+For downloading the specific version of the OneAgent, visit
+[this documentation](https://docs.dynatrace.com/docs/shortlink/api-deployment-get-oneagent-version). </br>
+To run tests in this mode, download 2 versions of installers you want along with the certificate and place them in 
+`test_dir/installers` directory. Then, you can run the tests.
+
+```console
+# Create directory `test_dir/installers` and place the installers and certificate in it
+$ mkdir -p test_dir/installers
+$ cp /path/and/name/of/both/installers test_dir/installers
+$ wget https://ca.dynatrace.com/dt-root.cert.pem -P test_dir/installers
+
+# Run tests on local machine
+$ sudo bash -c "source venv/bin/activate && pytest roles/oneagent/tests --linux_x86=localhost --preserve_installers"
+```