tests: replace the flat list of contexts with a hash

This way we can return more values without breaking callers.

tests/evaluate-basic.t

@@ -3,7 +3,7 @@ use strict;
 use Cwd;
 use Setup;
 
-(my $datadir, my $pgsql) = test_init();
+my %ctx = test_init();
 
 require Hydra::Schema;
 require Hydra::Model::DB;