Add config for specifying broken tests in debcargo.toml

This will reduce the amount of patches we need, and make the setup less brittle.

Instead of adding a patch that looks like this:

--- a/examples/in_tests.rs
+++ b/examples/in_tests.rs
@@ -41,6 +41,7 @@ mod tests {
     }

     #[test]
+    #[ignore = "broken in debian"]
     fn it_does_not_work() {
         init_logger();

We can instead add this line to debcargo.toml:

broken_tests = ["tests::it_does_not_work"]

and debcargo will add '-- --skip tests::it_does_not_work' to the Test-Command