trainer.test() should return the test score
🐛 Bug
trainer.test() has no return value. However, after testing one might like to have the test score returned, and saved.
Code sample
test_score = trainer.test()

Expected behavior
trainer.test() should return the final test score.
Environment
* CUDA:
	- GPU:
	- available:         False
	- version:           None
* Packages:
	- numpy:             1.18.5
	- pyTorch_debug:     False
	- pyTorch_version:   1.5.0
	- pytorch-lightning: 0.7.6
	- tensorboard:       2.2.2
	- tqdm:              4.46.1
* System:
	- OS:                Darwin
	- architecture:
		- 64bit
		-
	- processor:         i386
	- python:            3.7.7
	- version:           Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64

Additional context
Althought in some settings, test is only done immediately before publication and thus should not be used for model selection, there are other settings where validation is used for early stopping and test is used for final held-out evaluation. In these settings, being able to save the test score is important.