Add more build settings to test targets (#2106)

Motivation:

The test targets don't set the same Swift as the library targets.
There's no reason for them not to.

Modifications:

Apply default settings to test targets.

Result:

Test targets use Swift 6 language mode, explicit existential any, and
internal imports by default

Author: glbrntt

Package.swift

@@ -69,7 +69,8 @@ let targets: [Target] = [
     ],
     resources: [
       .copy("Configuration/Inputs")
-    ]
+    ],
+    swiftSettings: defaultSwiftSettings
   ),
 
   // In-process client and server transport implementations
@@ -84,7 +85,8 @@ let targets: [Target] = [
     name: "GRPCInProcessTransportTests",
     dependencies: [
       .target(name: "GRPCInProcessTransport")
-    ]
+    ],
+    swiftSettings: defaultSwiftSettings
   ),
 
   // Code generator library for protoc-gen-grpc-swift
@@ -97,7 +99,8 @@ let targets: [Target] = [
     name: "GRPCCodeGenTests",
     dependencies: [
       .target(name: "GRPCCodeGen")
-    ]
+    ],
+    swiftSettings: defaultSwiftSettings
   ),
 ]