Make TestNetworkConnectionTracker a singleton.

As more classes get migrated from NetworkChangeManager to
NetworkConnectionTracker, more and more tests need a
TestNetworkConnectionTracker instance set up, including many tests that
are pulling in the NetworkConnectionTracker indirectly, so shouldn't
need to set up the test instance themselves. To make sure all tests
have access to the test instance, this CL makes
TestNetworkConnectionTracker a singleton that is setup at the beginning
of each test suite, which ensures only one instance is ever created,
and it's torn down at the right time.

Bug: 868015
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I90ca6ddfa1669f476b03fa7e27230288dfc4101e
Reviewed-on: https://chromium-review.googlesource.com/1214081
Reviewed-by: Ilya Sherman <[email protected]>
Reviewed-by: Kristi Park <[email protected]>
Reviewed-by: Robert Sesek <[email protected]>
Reviewed-by: Doug Arnett <[email protected]>
Reviewed-by: Min Qin <[email protected]>
Reviewed-by: Sergey Ulanov <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Commit-Queue: Robbie McElrath <[email protected]>
Cr-Commit-Position: refs/heads/master@{#591463}
32 files changed