找到了这么一段
Changes in ADO.NET provider assembly loading
NHibernate 1.2 now uses Assembly.Load() instead of Assembly.LoadWithPartialName() to load driver assemblies. This means that it will no longer look for the highest-versioned assembly in the Global Assembly Cache (GAC), which was sometimes undesirable. Instead, it is now your responsibility to either put the provider assembly into the application directory, or add a qualifyAssembly element to the application configuration file, specifying the full name of the assembly.
For example, if you are using MySQL Connector/.NET, you should either put MySql.Data.dll into the application directory (or some other directory from where Assembly.Load() can pick it up), or put it in the GAC and add a qualifyAssembly element to the configuration file:
<qualifyAssembly partialName="MySql.Data" fullName="MySql.Data, Version=..., PublicKeyToken=..."/>
于是找到MySql.Data.dll，并拷贝到单元测试的目录下，上面的错误提示没了。