[ClientRuntime] Fixing spurious test failures in JsonSerializationTests

[brjohnstmsft]

Two of the JsonSerializationTests modify JsonConvert.DefaultSettings. They do this in order to validate that SafeJsonConvert does not rely on these settings.

Before this change, these tests would install a custom JSON converter and contract resolver that always throw. Unfortunately, other aspects of the test infrastructure rely on DefaultSettings, so this would sometimes result in unexpected exceptions being thrown.

The solution is to weaken the effect of changing DefaultSettings. Now, the SafeJsonConvert tests install a custom converter for their own Model class that produces incorrect results. Because it's targeted at that class specifically, it should have no side effects on other code that relies on DefaultSettings.

FYI @shahabhijeet @dsgouda

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