Check errors by identity, not by message text, in tests (closes #49)

Tests that asserted a failure by a fragment of its message now use
errors.Is: a refactor returning the wrong error, or wrapping with %v
instead of %w, now fails them. New tests return each exported error of
internal/vault and pkg/bip85 that no test returned, and check wrapped
causes (os.ErrNotExist, ErrMnemonicMismatch through GetSecret,
ErrInvalidPathComponent through DeriveBIP85Entropy). The 999-versions
test moves into package secret to name its unexported error. Checks of
errors no test can name keep their text; they are listed on the issue.

Model: opus-5-5

internal/secret/seunlocker_test.go

@@ -4,6 +4,8 @@
 package secret
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -106,6 +108,10 @@ func TestSecureEnclaveUnlockerGetIdentityMissingFile(t *testing.T) {
 	// GetIdentity should fail because the encrypted longterm key file is missing
 	identity, err := unlocker.GetIdentity()
 	assert.Nil(t, identity)
-	require.Error(t, err)
-	assert.Contains(t, err.Error(), "failed to read SE-encrypted long-term key")
+
+	var cause *os.PathError
+
+	require.ErrorAs(t, err, &cause)
+	require.ErrorIs(t, err, os.ErrNotExist)
+	assert.Equal(t, filepath.Join(dir, seLongtermFilename), cause.Path)
 }