I'm planning to setup an Oracle Standard Edition instance on a server in our test environment.
Do I need to purchase a license for this?
In general, yes. Once you have an application in production, every environment (dev, test, staging, production, etc.) needed to support that application needs to be licensed. As with all licensing advice, however, you need to get it in writing from Oracle Sales. Advice from some guy on the internet holds up about as well as you might expect if there is ever a question about whether you're properly licensed and/or if you're overpaying for licenses. And remember that agreements your company has with Oracle, laws in your country, etc. can change this general statement.
If you are a reasonably large customer or if you have some other leverage in the negotiation process, however, you can generally get the licenses for the test and development environments at a good discount from Oracle Sales.
The Oracle Software Investment Guide says the following:
Test/Staging - Test/staging environments are used to verify that new or customized code runs properly. This can be staged on separate servers or on the same servers used to run a development or production environment. Any Oracle software used in test/staging environment must be properly licensed with a Full Use license under an Oracle License and Services Agreement (OLSA) or other appropriate Oracle (or Oracle authorized reseller) license agreement. If a test/staging environment is maintained on the same server as a production or development environment, and that server is fully licensed for all relevant programs on a per Processor metric, then no additional licenses are required for the test/staging environment.
If you have a fully licensed server with a virtualized database running on it, you might want to consider adding a second virtual machine to the same server to run your test system on. It would save you from having to purchase an additional license if the resources can accommodate it.