GH-3530: Add exception handler to handle cases when the table already exists.

Partially revert part of the previous commit which changed the responisbility
of the SchemaExporter. Hopefully this fixes failing tests due to schema issues.

Author: David Ellingsworth

src/NHibernate.Test/NHSpecificTest/GH3530/Fixture.cs

@@ -38,13 +38,12 @@ protected override void OnTearDown()
 
 		protected override void CreateSchema()
 		{
-			// Let the SchemaExporter drop and create tables if needed.
-			base.CreateSchema();
-
 			CreateTable("Integer");
 			CreateTable("DateTime");
 			CreateTable("Double");
 			CreateTable("Decimal");
+
+			base.CreateSchema();
 		}
 
 		/// <summary>
@@ -83,6 +82,10 @@ private void CreateTable(string name)
 						cmd.ExecuteNonQuery();
 					}
 				}
+				catch (Exception ex)
+				{
+					Assert.Warn($"Creating the schema failed, assuming it already exists. {ex}");
+				}
 				finally
 				{
 					Sfi.ConnectionProvider.CloseConnection(cn);

src/NHibernate.Test/NHSpecificTest/GH3530/Mappings.hbm.xml

@@ -7,19 +7,19 @@
   not provide a mechanism of doing this, hence they have a
   schema-action of 'none'.
   -->
-  <class name="IntegerEntity" schema-action="drop">
+  <class name="IntegerEntity" schema-action="none">
     <id name="Id" generator="guid.comb" />
     <property name="DataValue" column="DataValue"/>
   </class>
-  <class name="DateTimeEntity" schema-action="drop">
+  <class name="DateTimeEntity" schema-action="none">
     <id name="Id" generator="guid.comb" />
     <property name="DataValue" column="DataValue"/>
   </class>
-  <class name="DoubleEntity" schema-action="drop">
+  <class name="DoubleEntity" schema-action="none">
     <id name="Id" generator="guid.comb" />
     <property name="DataValue" column="DataValue"/>
   </class>
-  <class name="DecimalEntity" schema-action="drop">
+  <class name="DecimalEntity" schema-action="none">
     <id name="Id" generator="guid.comb" />
     <property name="DataValue" column="DataValue"/>
   </class>