chore: Enable clippy::assertions_on_result_states

This lint disallows asserttions on is_ok()/is_err() in favor of either
using unwrap (so that at least if the test fails, we the failure message
will contain the actual failure reason instead of just "was not
ok/err"), or actually matching the specific variant.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

.cargo/config

@@ -11,6 +11,7 @@ rustflags = [
   "-Wmissing_debug_implementations",
   "-Wclippy::exit",
   "-Wclippy::tests_outside_test_module",
+  "-Wclippy::assertions_on_result_states"
 ]
 
 [net]