Hi,
thanks for the replies. I was digging into this a bit deeper regarding your answers and found out several facts. So to help others who face the same issues, here are my results:
1. The surefire tests are only executed in the install phase. Personally this is confusing, as I tried to build my shortened examples using "mvn test" and "mvn package" as I didn't wanted to install it. And no tests where executed. They are only executed when executing "mvn install". Maybe not a big deal and necessary to ensure correct test execution, but IMHO confusing. So you need to be aware of that when using Tycho.
2. With the answers of Mickael Istria and Mikhail Karkov I was able to find out what's going on. It is very important to know that the tests are executed against the packaged plugin jar. Because of this, the URL changes in test execution. While executing the test in the IDE the URL is a "file:/" URL, executing the test with Tycho it will be a "bundleclass:/" URL. Using Java File or NIO API, the file can not be loaded with conventional methods if the URL starts with "bundleclass".
The solution mentioned by Mikhail Karkov is a possible workaround, but IMHO it is not a very elegant one. Because in that case I am forced to implement my test cases with a quite hard dependency to the environment. If I simply want to test some file util classes, I don't want to deal with that.