[pigeon][reland]enable treat warnings as errors for swift code in unit test (#3901)

`as [Any?]` here is actually not a "cast", but a "type annotation". An equivalent fix can be: 

```
let arg: [Any?] = [nil]
let inputEncoded = binaryMessenger.codec.encode(arg)
```

Fixes
```
heterogeneous collection literal could only be inferred to '[Any?]'; add explicit type annotation if this is intentional
```

Which was introduced in #3889 and later got reverted. 

See flutter/flutter#126006 for more details about some interesting research on this warning. 

*List which issues are fixed by this PR. You must list at least one issue.*

Fixes flutter/flutter#126006

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: hellohuanlin

packages/pigeon/platform_tests/test_plugin/example/ios/Runner.xcodeproj/project.pbxproj

@@ -542,6 +542,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -557,6 +558,7 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -572,6 +574,7 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};

packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/NullableReturnsTests.swift

@@ -40,7 +40,7 @@ class NullableReturnsTests: XCTestCase {
     NullableArgHostApiSetup.setUp(binaryMessenger: binaryMessenger, api: api)
     XCTAssertNotNil(binaryMessenger.handlers[channel])
 
-    let inputEncoded = binaryMessenger.codec.encode([nil])
+    let inputEncoded = binaryMessenger.codec.encode([nil] as [Any?])
 
     let expectation = XCTestExpectation(description: "callback")
     binaryMessenger.handlers[channel]?(inputEncoded) { _ in