[c#] Increase unit test's UnsafeBufferSize
[c#] Increase unit test's UnsafeBufferSize

The GenericInheritance test case generates random payloads. Sometimes
these are larger than the current UnsafeBufferSize (192 * 1024 bytes),
so the test fails when writing to an OutputPointer, which cannot
automatically grow its buffer.

To compute the new value of 4 MiB, I changed RandomReader to always
return the maximum length for strings and containers and to return the
largest numbers possible. When transcoding to the Simple Binary
protocol, the payload was ~168KiB shy of 4 MiB, so I picked 4 MiB. The
other protocols were in the 3.8 MiB range.

Closes https://github.com/Microsoft/bond/pull/267

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