can you try to run svn checkout once manually to your URL https://yoururl/trunk C:\ant-1.8.1\Test_Checkout using command line
and accept certificate.
Or as @AndrewSpear says below
Rather than checking out manually run svn list https://your.repository.url from Terminal (Mac) / Command Line (Win) to get the option to accept the certificate permanently
svn will ask you for confirmation. accept it permanently.
After that this should work for subsequent requests from ant script.