Rename the swift-testing entry point. (#7137)

This PR renames the entry point function used by SwiftPM to call into
swift-testing such that it does not need to be SPI. By marking it SPI,
we unintentionally made it very difficult to use with a toolchain that
does not include private module information.

swift-testing will continue to export the old symbol name as used by
SwiftPM until this change has firmly landed in toolchains, after which
we will remove the old function.

See: swiftlang/swift-testing#141

Author: grynspan

Sources/Build/BuildOperationBuildSystemDelegateHandler.swift

@@ -219,13 +219,13 @@ final class TestEntryPointCommand: CustomLLBuildCommand, TestBuildCommand {
             stream.send(
                 #"""
                 #if canImport(Testing)
-                @_spi(SwiftPackageManagerSupport) import Testing
+                import Testing
                 #endif
 
                 @main struct Runner {
                     static func main() async {
                 #if canImport(Testing)
-                        await Testing.swiftPMEntryPoint() as Never
+                        await Testing.__swiftPMEntryPoint() as Never
                 #endif
                     }
                 }