runtime: allow omitting virtual PCs from runtime.CallersFrames input

This makes CL 561635's test pass without any changes to the
traceback textual format.

The test in this CL is copied identically from CL 561635.

Change-Id: I5130abdfefd9940f98f20c283cca6cd159e37617
Reviewed-on: https://go-review.googlesource.com/c/go/+/571798
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/crash_test.go

@@ -14,6 +14,7 @@ import (
 	"internal/testenv"
 	tracev2 "internal/trace/v2"
 	"io"
+	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -28,7 +29,19 @@ import (
 
 var toRemove []string
 
+const entrypointVar = "RUNTIME_TEST_ENTRYPOINT"
+
 func TestMain(m *testing.M) {
+	switch entrypoint := os.Getenv(entrypointVar); entrypoint {
+	case "crash":
+		crash()
+		panic("unreachable")
+	default:
+		log.Fatalf("invalid %s: %q", entrypointVar, entrypoint)
+	case "":
+		// fall through to normal behavior
+	}
+
 	_, coreErrBefore := os.Stat("core")
 
 	status := m.Run()