Add soak test for memory usage with external tilesets

[azrogers]

To test out CesiumGS/cesium-native#876, I've created a long-running soak test to evaluate the memory usage of cesium-native over a long period. The test consists of loading the Google P3DT and flying to random locations around the globe over and over again for twelve straight hours. There's a few issues with this test as it stands:

1. Because of CesiumGS/cesium-native#749, the test can only actually run for three hours total, since after that the Google P3DT token will need to be refreshed and we don't currently handle this possibility. I can't think of a better dataset than P3DT to run this test with, so do we fix this bug instead?
2. After each flight it chooses a new position in the opposite quadrant of the earth to fly to (if it's in the northern and western hemispheres, it will choose a point in the southern and eastern hemispheres, for example). This means most of the points it chooses are in the middle of nowhere (fields, ocean) which aren't exactly a great test. It would be great to choose random locations that have lots of buildings and therefore content to load instead, but I can't think of a way to do this other than working from a set list of locations.

[csciguy8]

There's a few issues with this test as it stands... the test can only actually run for three hours total... I can't think of a better dataset than P3DT to run this test with, so do we fix this bug instead?

Hitting the token refresh interval doesn't sound like an issue with this PR. If the soak test does fail because of token issues, sounds like it's doing the job perfectly (finding any issue when running a long time).

[kring]

(deleted this comment because I added it to the wrong issue)