Strict concurrency enablement for some test targets (#3105)

Motivation:

A few more low-hanging-fruit test targets.

Modifications:

Enable strict concurrency for:

- NIOFoundationCompatTests
- NIOSingletonsTests

Result:

We're knocking down those targets, one by one.

Package.swift

@@ -521,15 +521,17 @@ let package = Package(
             dependencies: [
                 "NIOCore",
                 "NIOFoundationCompat",
-            ]
+            ],
+            swiftSettings: strictConcurrencySettings
         ),
         .testTarget(
             name: "NIOTests",
             dependencies: ["NIO"]
         ),
         .testTarget(
             name: "NIOSingletonsTests",
-            dependencies: ["NIOCore", "NIOPosix"]
+            dependencies: ["NIOCore", "NIOPosix"],
+            swiftSettings: strictConcurrencySettings
         ),
         .testTarget(
             name: "NIOFileSystemTests",