fix: run retry integration tests

SmithyRetriesTests test target was accidentally removed from Package.swift
in smithy-lang#692, so this commit adds it back. The test was also not working properly,
because Context.partitionID's attribute key name used by RetryMiddleware
was different from the one being set in the test. This caused the test to
exit early when partitionID wasn't found.

Package.swift

@@ -240,5 +240,9 @@ let package = Package(
             name: "SmithyWaitersAPITests",
             dependencies: ["Smithy", "SmithyWaitersAPI"]
         ),
+        .testTarget(
+            name: "SmithyRetriesTests",
+            dependencies: ["ClientRuntime", "SmithyRetriesAPI", "SmithyRetries", "SmithyTestUtil"]
+        ),
     ].compactMap { $0 }
 )

Tests/SmithyRetriesTests/RetryIntegrationTests.swift

@@ -17,7 +17,6 @@ import SmithyRetriesAPI
 
 // This test class reproduces the "Standard Mode" test cases defined in "Retry Behavior 2.0"
 final class RetryIntegrationTests: XCTestCase {
-    private let partitionIDKey = AttributeKey<String>(name: "PartitionID")
     private let partitionID = "partition"
 
     private var context: Context!
@@ -28,7 +27,7 @@ final class RetryIntegrationTests: XCTestCase {
     private func setUp(availableCapacity: Int, maxCapacity: Int, maxRetriesBase: Int, maxBackoff: TimeInterval) async {
         // Setup the HTTP context, used by the retry middleware
         context = Context(attributes: Attributes())
-        context.attributes.set(key: partitionIDKey, value: partitionID)
+        context.partitionID = partitionID
         context.socketTimeout = 60.0
         context.estimatedSkew = 30.0
 
@@ -239,7 +238,12 @@ private class TestOutputHandler: Handler {
     }
 
     func verifyResult(atEnd: Bool = true) async throws {
-        guard let testStep = latestTestStep else { return }
+        guard let testStep = latestTestStep else {
+            if atEnd {
+                XCTFail("No test steps were run! Encountered error: \(String(describing: finalError!))")
+            }
+            return
+        }
 
         // Test available capacity
         let availableCapacity = await quota.availableCapacity