Remove Bundle-RequiredExecutionEnvironment from manifest of test bundles

The test bundles use Java 8 and would require a BREE of JavaSE-1.8.
However, Eclipse Indigo and Juno (falsely) declare they are only
compatible with execution environments up to Java 7 and refuse to start
bundles declaring a BREE of JavaSE-1.8. If the declaration is ommitted
everything works fine.