Fix prop_state_machine cannot find value `config`

[sameer]

When #![proptest_config(..)] is not provided, the macro should use the default proptest config. This PR fixes an issue that prevents this from working -- there's a reference to config that should be Default::default() instead:

error[E0425]: cannot find value `config` in this scope
     |
2132 | /     prop_state_machine! {
2133 | |         #[test]
2134 | |         fn run(sequential 1..20 => MyTest);
2135 | |     }
     | |_____^ not found in this scope
     |
     = note: this error originates in the macro `prop_state_machine` (in Nightly builds, run with -Z macro-backtrace for more info)

[sameer]

Just dropping a note that the test failures are unrelated. It's because the checks use Rust 1.64 but regex-automata v0.4.1 expects 1.65+.

[tzemanovic]

Just dropping a note that the test failures are unrelated. It's because the checks use Rust 1.64 but regex-automata v0.4.1 expects 1.65+.

yup, it's fixed on master now - merging