Restore field names with f prefix. Fixes #960.

The prefix has been removed by df00d5eced3a7737b88de0f6f9e3673f0cf88f88,
because we changed the coding style. Unfortunately IntelliJ IDEA reads
ClassRequest.fTestClass and FilterRequest.fFilter via reflection. (See
http://grepcode.com/file/repository.grepcode.com/java/ext/com.jetbrains/intellij-idea/13.0.0/com/intellij/junit4/JUnit4IdeaTestRunner.java/ ) This makes it impossible to
start tests by using IntelliJ IDEA.

There is already an issue at JetBrains:
[IDEA-127349 ](http://youtrack.jetbrains.com/issue/IDEA-127349) We can
revert the fix when this ticket is solved and most users are using an
IntelliJ version with the fix.