internal/testkeys: use fixed keys in unit tests

[davidweisse]

We can speed up unit tests, and make them more deterministic by using fixed keys, instead of generating them at runtime. A number of fixed keys are stored in the internal/testkeys package in Base64 encoded PKCS8 format, which can be used in unit tests with the testkeys.New function.

[burgerdev]

Suggested change

	if _, ok := key.(*T); !ok {
	return nil, fmt.Errorf("invalid key type: %T", key)
	}
	return key.(*T), nil
	typedKey, ok := key.(*T);
	if !ok {
	return nil, fmt.Errorf("invalid key type: %T", key)
	}
	return typedKey, nil

[burgerdev]

I think this would be easier to use if the signature was

func New[T crypto.PrivateKey](t *testing.T, keyString EncodedKeyString) *T

and the function would fail the test if the key does not decode.