Refactor out TestNetworkQualityEstimator

Refactor out TestNetworkQualityEstimator from
n_q_e_unittest.cc to a separate file, so it can be used
by other classes in //net/nqe (Other test classes coming soon
in a forthcoming CL).

BUG=490870

Review-Url: https://codereview.chromium.org/2264203002
Cr-Commit-Position: refs/heads/master@{#416696}
6 files changed