chore: add nextest cover profile, update backward-compat imports

Nextest cover profile for cargo llvm-cov integration.
Update existing test imports from load_workflow_from_str to
load_single_workflow_from_str for backward compatibility.

wfe-yaml/tests/shell.rs

@@ -7,14 +7,14 @@ use wfe::{WorkflowHostBuilder, run_workflow_sync};
 use wfe_core::test_support::{
     InMemoryLockProvider, InMemoryPersistenceProvider, InMemoryQueueProvider,
 };
-use wfe_yaml::load_workflow_from_str;
+use wfe_yaml::load_single_workflow_from_str;
 
 async fn run_yaml_workflow_with_data(
     yaml: &str,
     data: serde_json::Value,
 ) -> wfe::models::WorkflowInstance {
     let config = HashMap::new();
-    let compiled = load_workflow_from_str(yaml, &config).unwrap();
+    let compiled = load_single_workflow_from_str(yaml, &config).unwrap();
 
     let persistence = Arc::new(InMemoryPersistenceProvider::new());
     let lock = Arc::new(InMemoryLockProvider::new());