ExpressionTest : Use error tolerant loading to load legacy files.
ExpressionTest : Use error tolerant loading to load legacy files.

These tests were failing because we had serialised unwanted connections into the `Expression::inPlug()` from the `Node::userPlug()`, due to the automatic connection tracking behaviour exhibited by plugs. Now `Node::userPlug()` is a Plug, it is no longer suitable for connection into a ValuePlug like `Expression::inPlug()` and an error occurs during loading.

andrewkaufman/gaffer,hradec/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,boberfly/gaffer,GafferHQ/gaffer,chippey/gaffer,johnhaddon/gaffer,boberfly/gaffer,chippey/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,hradec/gaffer,chippey/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,boberfly/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,boberfly/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,chippey/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer