Use SQL Compact 4.0 (download both SqlCE and tools by web platform installer) – EF Code first has direct support for that. The only difference will be that your application will use connection string to big SQL Server:
<add name="MyDbContext"
provider="System.Data.SqlClient"
connectionString=
"Data Source=...;InitialCatalog=...;Integrated Security=SSPI" />
and your tests will use connection string to SQL Compact:
<add name="MyDbContext"
provider="System.Data.SqlServerCe.4.0"
connectionString="Data Source=Database.sdf" />