testing: avoid Goexit without unwind support

Goexit cannot run deferred functions on architectures without panic
unwinding. Query runtime.supportsRecover before calling it so unsupported
targets report an error instead of leaving the test runner blocked forever.

src/testing/testing.go

@@ -25,6 +25,7 @@ import (
 	"time"
 	"unicode"
 	"unicode/utf8"
+	_ "unsafe"
 )
 
 // Testing flags.
@@ -215,7 +216,10 @@ func (c *common) Failed() bool {
 func (c *common) FailNow() {
 	c.Fail()
 	c.finished = true
-	runtime.Goexit()
+	if supportsRecover() {
+		runtime.Goexit()
+	}
+	c.Error("FailNow is incomplete, requires runtime.Goexit()")
 }
 
 // log generates the output.
@@ -287,13 +291,19 @@ func (c *common) Skipf(format string, args ...any) {
 func (c *common) SkipNow() {
 	c.skip()
 	c.finished = true
-	runtime.Goexit()
+	if supportsRecover() {
+		runtime.Goexit()
+	}
+	c.Error("SkipNow is incomplete, requires runtime.Goexit()")
 }
 
 func (c *common) skip() {
 	c.skipped = true
 }
 
+//go:linkname supportsRecover runtime.supportsRecover
+func supportsRecover() bool
+
 // Skipped reports whether the test was skipped.
 func (c *common) Skipped() bool {
 	return c.skipped