runtime, testing: support Goexit, SkipNow, FailNow

Keep a pending Goexit separate from the active panic state so recovered
deferred panics do not cancel the original Goexit unwind. Goexit should
also ignore -panic=trap, since it is not a panic.

Wire testing.FailNow and SkipNow through runtime.Goexit, and run tests
and benchmarks in goroutines. This lets Goexit terminate the user
function without skipping test bookkeeping. Add Goexit/recover coverage
and enable crypto/ecdh in the Linux stdlib test list.

testdata/testing.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"flag"
 	"io"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -25,11 +26,21 @@ func TestBar(t *testing.T) {
 		t.Log("after failed")
 	})
 	t.Run("Bar3", func(t *testing.T) {})
+	if runtime.GOARCH != "wasm" {
+		t.Run("Bar4", func(t *testing.T) {
+			t.Fatal("fatal")
+			t.Log("after fatal")
+		})
+		t.Run("Bar5", func(t *testing.T) {
+			t.SkipNow()
+			t.Error("after skip")
+		})
+	}
 	t.Log("log Bar end")
 }
 
 func TestAllLowercase(t *testing.T) {
-	names := []string {
+	names := []string{
 		"alpha",
 		"BETA",
 		"gamma",