Fixed intermittent unit test failures. (#7651)

The problem is that SurfacePoint doesn't have default values when constructed, and a few of the unit tests weren't setting m_position because the values weren't strictly needed for the test. However, the SurfacePointList checks them for validity and asserted when they couldn't be found, which was hit-and-miss due to it being uninitialized memory that was usually but not always 0xCCCCCCCC.
Improved the assert message and set the position values everywhere.

Signed-off-by: Mike Balfour <82224783+mbalfour-amzn@users.noreply.github.com>

Gems/SurfaceData/Code/Source/SurfacePointList.cpp

@@ -32,7 +32,11 @@ namespace SurfaceData
             inPositionIndex = (inPositionIndex + 1) % m_inputPositions.size();
         }
 
-        AZ_Assert(foundMatch, "Couldn't find input position!");
+        AZ_Assert(
+            foundMatch,
+            "Couldn't find input position: (%0.7f, %0.7f, %0.7f), m_lastInputPositionIndex = %zu, m_inputPositions.size() = %zu",
+            inPosition.GetX(), inPosition.GetY(), inPosition.GetZ(), m_lastInputPositionIndex, m_inputPositions.size());
+
         m_lastInputPositionIndex = inPositionIndex;
         return inPositionIndex;
     }