[Backend Tester] Clean up report output

[GregoryComer]

Update the report output to be a bit cleaner. I've made the following changes:

• Round decimal values to 3 decimal places.
• Remove mean signed deviation and L2 norm. We have SNR, mean absolute error, and max element-wise error. I think that's sufficient.
• Rename SQNR to SNR. Not all tests are quantized and we're using it as a general measure of tensor-wise error.
• Split out the result column into pass/skip/fail and a detailed reason. This should be easier to parse at a glance.

Example output (not sure markdown formatting is weird here...):

Test ID	Test Case	Flow	Params	Result	Result Detail	Delegated	Quantize Time (s)	Lower Time (s)	Delegated Nodes	Undelegated Nodes	Delegated Ops	Undelegated Ops	PTE Size (Kb)	Output 0 Error Max	Output 0 Error MAE	Output 0 SNR	Output 1 Error Max	Output 1 Error MAE	Output 1 SNR
test_add_dtype_float32_xnnpack	test_add_dtype	xnnpack	{'dtype': torch.float32}	Pass		TRUE		3.747	1	0	{'aten::add.Tensor': 1}	{}	1.6	0	0	inf
test_add_dtype_float32_xnnpack_static_int8_per_channel	test_add_dtype	xnnpack_static_int8_per_channel	{'dtype': torch.float32}	Pass		TRUE	0.66	0.818	7	0	{'aten::add.Tensor': 1, 'quantized_decomposed::dequantize_per_tensor': 3, 'quantized_decomposed::quantize_per_tensor': 3}	{}	2	0	0	inf