internal/stub: move from container

This package solves a very specific stubbing use case, in a less than elegant manner.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/stub/errors_test.go

@@ -0,0 +1,42 @@
+package stub_test
+
+import (
+	"errors"
+	"syscall"
+	"testing"
+
+	"hakurei.app/internal/stub"
+)
+
+func TestUniqueError(t *testing.T) {
+	t.Parallel()
+
+	t.Run("format", func(t *testing.T) {
+		t.Parallel()
+		want := "unique error 2989 injected by the test suite"
+		if got := stub.UniqueError(0xbad).Error(); got != want {
+			t.Errorf("Error: %q, want %q", got, want)
+		}
+	})
+
+	t.Run("is", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("type", func(t *testing.T) {
+			t.Parallel()
+			if errors.Is(stub.UniqueError(0), syscall.ENOTRECOVERABLE) {
+				t.Error("Is: unexpected true")
+			}
+		})
+
+		t.Run("val", func(t *testing.T) {
+			t.Parallel()
+			if errors.Is(stub.UniqueError(0), stub.UniqueError(1)) {
+				t.Error("Is: unexpected true")
+			}
+			if !errors.Is(stub.UniqueError(0xbad), stub.UniqueError(0xbad)) {
+				t.Error("Is: unexpected false")
+			}
+		})
+	})
+}