Use mock JDK in compiler tests where full JDK is not needed

- move some of boxWithStdlib tests under fullJdk/ directory, where they will be
  compiled against the full JDK
- introduce FULL_JDK in-text directive for the reflection test as only 4 tests
  out of 654 needed the full JDK

compiler/testData/codegen/boxAgainstJava/reflection/mapping/javaFields.kt

@@ -1,3 +1,4 @@
+// FULL_JDK
 import javaFields as J
 
 import java.lang.reflect.*