Fix terrain tests by properly initializing low detail terrain to zero height.

Also remove PI heightmap test as new heightmaps only have two significant digits.

OpenSim/Framework/TerrainData.cs

@@ -274,6 +274,7 @@ namespace OpenSim.Framework
             m_taint = new bool[SizeX / Constants.TerrainPatchSize, SizeY / Constants.TerrainPatchSize];
             // m_log.DebugFormat("{0} new by dimensions. sizeX={1}, sizeY={2}, sizeZ={3}", LogHeader, SizeX, SizeY, SizeZ);
             ClearTaint();
+            ClearLand(0f);
         }
 
         public HeightmapTerrainData(short[] cmap, float pCompressionFactor, int pX, int pY, int pZ) : this(pX, pY, pZ)