Move tests/mlflow/ to tests/

[andrewmchen]

We should move the tests from tests/mlflow/ to tests/ since Python 2 will get confused about relative imports from test files located in tests/.

For example, if I have a file tests/test_foo.py. I cannot do

import mlflow.cli

since in Python 2 it will try to import the tests/mlflow/ package first.

[tomasatdatabricks]

lgtm!

[codecov-io]

Codecov Report

Merging #196 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   50.89%   50.91%   +0.02%     
==========================================
  Files          93       93              
  Lines        4761     4761              
==========================================
+ Hits         2423     2424       +1     
+ Misses       2338     2337       -1

Impacted Files	Coverage Δ
mlflow/tracking/__init__.py	82.88% <0%> (+0.53%)	⬆️

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