Create testing sever classes that use TestAmple to simulate flaky conditional mutations

[keith-turner]

Following the pattern in #4643 it may be possible to create test server classes like FlakyAmpleManager and FlakyAmpleTserver that have the following properties.

• The sever context in these classes returns TestAmple
• The TestAmpl impl returned will randomly report an uknown status for conditional mutations and when it does this randomly sometimes not write the mutation.

Doing this would allow testing of handling of UNKNOWN status for conditional mutations. This situation never happens in normal testing because it requires a tablet server to die or have a network problem. So this leaves a lot of code for handling UNKNOWN not well tested.