Reorganize tests

[jeremyliweishih]

Organize tests into:

• automl_tests
• demo_tests
• objective_tests
• pipeline_tests
• preprocessing_tests
• problem_type_tests

[codecov]

Codecov Report

Merging #93 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   92.54%   92.54%           
=======================================
  Files          47       47           
  Lines        1248     1248           
=======================================
  Hits         1155     1155           
  Misses         93       93

Impacted Files	Coverage Δ
evalml/tests/objective_tests/test_lead_scoring.py	100% <ø> (ø)
evalml/tests/demo_tests/test_datasets.py	100% <ø> (ø)
...s/preprocessing_tests/test_detect_label_leakage.py	100% <ø> (ø)
...l/tests/pipeline_tests/test_logistic_regression.py	100% <ø> (ø)
evalml/tests/automl_tests/test_autoregressor.py	100% <ø> (ø)
evalml/tests/automl_tests/test_autoclassifier.py	100% <ø> (ø)
evalml/tests/pipeline_tests/test_xgboost.py	100% <ø> (ø)
evalml/tests/pipeline_tests/test_rf.py	100% <ø> (ø)
...alml/tests/objective_tests/test_fraud_detection.py	100% <ø> (ø)
evalml/tests/objective_tests/test_objectives.py	100% <ø> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 93c8adb...beda92f. Read the comment docs.

[kmax12]

LGTM