Add SellerEmail variable for testing data.

There are some situations were having the email address of the seller is
useful for testing.  This change allows for it as a parameter to the
tests.

auth.sample.3token

@@ -1,3 +1,4 @@
 Username  = test1_api.mydomain.tld
 Password  = XXXXXXXXXXXXXXXX
 Signature = XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+SellerEmail = email-of-business-test-account@mydomain.tld

auth.sample.cert

@@ -2,3 +2,4 @@ Username = test1_api.mydomain.tld
 Password = myapipassword
 CertFile = /www/var/cert_key_pem.txt
 KeyFile  = /www/var/cert_key_pem.txt
+SellerEmail = email-of-business-test-account@mydomain.tld

t/API.pl

@@ -26,7 +26,7 @@ sub do_args {
 
     my @variables = qw( Username Password Signature Subject timeout
         CertFile KeyFile PKCS12File PKCS12Password
-        BuyerEmail
+        BuyerEmail SellerEmail
     );
 
     my %patterns = ();