support testing/synctest when a non-test package imports "testing" too

As spotted by scripts/check-third-party.sh, it's possible to import
the testing package without using `go test`, so our previous solution
to only load testing/synctest when running `go test` was not enough.

Add a regression test via stdimporter in gogarble.txtar.

go_std_tables.go

@@ -87,8 +87,10 @@ var runtimeAndLinknamed = []string{
 	"unique",                        // go1.25
 	"weak",                          // go1.25
 	// The net package linknames to the runtime, not the other way around.
-	// TODO: support this automatically via our script.
 	"net",
+	// The testing package uses a //go:linkname directive pointing to testing/synctest,
+	// but it doesn't import the package, presumably to avoid an import cycle.
+	"testing/synctest",
 }
 
 var compilerIntrinsics = map[string]map[string]bool{

scripts/gen_go_std_tables.go

@@ -53,8 +53,10 @@ var runtimeAndLinknamed = []string{
 	"{{ $path.String }}", // {{ $path.GoVersionLang }}
 {{- end }}
 	// The net package linknames to the runtime, not the other way around.
-	// TODO: support this automatically via our script.
 	"net",
+	// The testing package uses a //go:linkname directive pointing to testing/synctest,
+	// but it doesn't import the package, presumably to avoid an import cycle.
+	"testing/synctest",
 }
 
 var compilerIntrinsics = map[string]map[string]bool{

shared.go

@@ -279,23 +279,12 @@ func appendListedPackages(packages []string, mainBuild bool) error {
 		// Similar flags to what go/packages uses.
 		"-json", "-export", "-compiled", "-e",
 	}
-	if len(packages) == 0 {
-		// As we may append to the packages slice below,
-		// ensure that zero packages still means "the current directory".
-		packages = append(packages, ".")
-	}
 	if mainBuild {
 		// When loading the top-level packages we are building,
 		// we want to transitively load all their dependencies as well.
 		// That is not the case when loading standard library packages,
 		// as runtimeAndLinknamed already contains transitive dependencies.
 		args = append(args, "-deps")
-		if slices.Contains(sharedCache.ForwardBuildFlags, "-test") {
-			// The testing package uses a //go:linkname directive pointing to testing/synctest,
-			// but it doesn't import the package, presumably to avoid an import cycle.
-			// For the linkname to obfuscate correctly, we need to list both when using `go test`.
-			packages = append(packages, "testing/synctest")
-		}
 	}
 	args = append(args, garbleBuildFlags...)
 	args = append(args, sharedCache.ForwardBuildFlags...)

testdata/script/gogarble.txtar

@@ -91,6 +91,7 @@ import (
 	"runtime"
 	"runtime/debug"
 	"syscall"
+	"testing"
 	"time"
 )
 
@@ -107,6 +108,7 @@ func main() {
 		debug.WriteHeapDump(1)
 		time.Now()
 		syscall.Listen(0, 1)
+		testing.MainStart(nil, nil, nil, nil, nil)
 	}
 }