Various cleanups, particularly to new G Challenges.

LambdaLab/test/exercises/C_DefaultMethods.java

LambdaLab/test/exercises/E_IntermediateStreams.java

LambdaLab/test/exercises/G_Challenges.java

LambdaLab/test/solutions/C_DefaultMethods.java

LambdaLab/test/solutions/E_IntermediateStreams.java

LambdaLab/test/solutions/G_Challenges.java