Enable upcoming feature 'InferIsolatedConformances' and fix issues it reveals (#1126)

This enables the `InferIsolatedConformances` upcoming Swift feature from
[SE-0470: Global-actor isolated
conformances](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0470-isolated-conformances.md)
and fixes a couple of pre-existing issues it revealed.

I confirmed these code changes still build successfully using a Swift
6.1 toolchain.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Package.swift

@@ -288,6 +288,8 @@ extension Array where Element == PackageDescription.SwiftSetting {
       // new-enough toolchain.
       .enableExperimentalFeature("AllowUnsafeAttribute"),
 
+      .enableUpcomingFeature("InferIsolatedConformances"),
+
       // When building as a package, the macro plugin always builds as an
       // executable rather than a library.
       .define("SWT_NO_LIBRARY_MACRO_PLUGINS"),

Sources/Testing/Running/Configuration.TestFilter.swift

@@ -514,7 +514,7 @@ extension Configuration.TestFilter.Kind {
 
 /// A protocol representing a value which can be filtered using
 /// ``Configuration/TestFilter-swift.struct``.
-private protocol _FilterableItem {
+private protocol _FilterableItem: Sendable {
   /// The test this item represents.
   var test: Test { get }
 

Tests/TestingMacrosTests/TestSupport/Parse.swift

@@ -19,7 +19,7 @@ import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
 import SwiftSyntaxMacroExpansion
 
-fileprivate let allMacros: [String: any Macro.Type] = [
+fileprivate let allMacros: [String: any (Macro & Sendable).Type] = [
   "expect": ExpectMacro.self,
   "require": RequireMacro.self,
   "requireAmbiguous": AmbiguousRequireMacro.self, // different name needed only for unit testing

cmake/modules/shared/CompilerSettings.cmake

@@ -17,7 +17,8 @@ add_compile_options(
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend ExistentialAny>"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InternalImportsByDefault>"
-  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend MemberImportVisibility>")
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend MemberImportVisibility>"
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InferIsolatedConformances>")
 
 # Platform-specific definitions.
 if(APPLE)