Refactor CEC SDM fit test

[Peque]

It feels like test_fit_sdm_cec_sam() could be split into two:

• Check for expected vs. actual output
• Check for exception when the parameters cannot be estimated

The code under the with statement has no relation with the code above.

[cwhanse]

This is a good suggestion. I'll roll the changes in #956 into #708 which refactors all of pvlib.ivtools. Please leave both #956 and #957 open for now.

[cwhanse]

Closed via #708, test function split into test_fit_cec_sam and test_fit_cec_sam_estimation_failure