gocryptfs -speed: call testing.Init() to fix panic

Looks like I should have been calling testing.Init()
all along. From https://pkg.go.dev/testing#Init :

> Init is only needed when calling functions such as
> Benchmark without using "go test".

Fixes: #789
Relates-to: golang/go#62666

internal/speed/speed.go

@@ -48,6 +48,7 @@ func Run() {
 		{name: cryptocore.BackendXChaCha20Poly1305OpenSSL.String(), f: bStupidXchacha, preferred: stupidgcm.PreferOpenSSLXchacha20poly1305()},
 		{name: cryptocore.BackendXChaCha20Poly1305.String(), f: bXchacha20poly1305, preferred: !stupidgcm.PreferOpenSSLXchacha20poly1305()},
 	}
+	testing.Init()
 	for _, b := range bTable {
 		fmt.Printf("%-26s\t", b.name)
 		mbs := mbPerSec(testing.Benchmark(b.f))