Fix C# unit tests (#818)

Summary:
This consists of several fixes:
  - Some of the C++ functions called by the tests were missing `WIN_EXPORT`
  - It looks like Yoga was changed to round up after `TestMeasureFuncWithFloat` was written. Here's the function call that results in the rounding: https://github.com/facebook/yoga/blob/357ca78f9f7bd86298ef00a1426cac5ca2041d3b/yoga/Yoga.cpp#L4019-L4026
  - The format of the result of the `Print` method was changed after `TestPrint` was written.
Pull Request resolved: facebook/yoga#818

Differential Revision: D10282902

Pulled By: shergin

fbshipit-source-id: a5fd732a7dbff1a704cbafbfc95ae3e0c0a0cdd8

Yoga.h

@@ -153,11 +153,11 @@ WIN_EXPORT void YGNodeCopyStyle(
     const YGNodeRef dstNode,
     const YGNodeRef srcNode);
 
-void* YGNodeGetContext(YGNodeRef node);
-void YGNodeSetContext(YGNodeRef node, void* context);
+WIN_EXPORT void* YGNodeGetContext(YGNodeRef node);
+WIN_EXPORT void YGNodeSetContext(YGNodeRef node, void* context);
 void YGConfigSetPrintTreeFlag(YGConfigRef config, bool enabled);
 YGMeasureFunc YGNodeGetMeasureFunc(YGNodeRef node);
-void YGNodeSetMeasureFunc(YGNodeRef node, YGMeasureFunc measureFunc);
+WIN_EXPORT void YGNodeSetMeasureFunc(YGNodeRef node, YGMeasureFunc measureFunc);
 YGBaselineFunc YGNodeGetBaselineFunc(YGNodeRef node);
 void YGNodeSetBaselineFunc(YGNodeRef node, YGBaselineFunc baselineFunc);
 YGDirtiedFunc YGNodeGetDirtiedFunc(YGNodeRef node);