Fix a warning when testing (#531)

Fixes a warning missed in https://github.com/apple/container/pull/516.

Sources/NativeBuilder/ContainerBuildParser/Docker/DockerInstructionVisitor.swift

@@ -24,7 +24,7 @@ protocol InstructionVisitor {
     func visit(_ label: LabelInstruction) throws
     func visit(_ expose: ExposeInstruction) throws
     func visit(_ arg: ArgInstruction) throws
-    // These instructions have to perform the ARG substitution checked in `testSimpleDockerfileArgInInstructions()`:
+    // TODO: These instructions have to perform the ARG substitution checked in `testSimpleDockerfileArgInInstructions()`:
     // - ADD
     // - ENV
     // - STOPSIGNAL

Tests/NativeBuilderTests/ContainerBuildIRTests/GraphBuilderTests.swift

@@ -239,7 +239,7 @@ struct GraphBuilderTests {
         #expect(graphBuilder.substituteArgs("a=${UNDEFINED:+value} b=${DEFINED:+value} c=${EMPTY:+value}", inFromContext: true) == "a= b=value c=")
     }
 
-    @Test(.serialized) func testPredefinedArgVariables() throws {
+    @Test func testPredefinedArgVariables() throws {
         let graphBuilder = GraphBuilder()
 
         var originalValues: [String: String?] = [:]