Feature: Add common test steps that are rendered into all generated tests #40

Problem

Some kuttl test steps are identical across every test in an operator — the clearest example is a
teardown step that deletes the product custom resources before the namespace is removed (see #2).
Today beku renders only each test's own template directory, so a common step has to be copied into
every tests/templates/kuttl/<test>/ directory and kept in sync by hand.

Proposal

Teach beku to render a shared directory into every generated test case, in addition to that
test's own steps.

• Files in a shared/ directory next to the test templates (i.e. tests/templates/kuttl/shared) are
  rendered into every generated test case.
• The directory is templated exactly like regular steps: .j2/.jinja2 files are rendered (with
  NAMESPACE and lookup available), others are copied verbatim.
• Location overridable with a new -c/--common_dir CLI flag.
• No-op unless the directory exists — repositories that don't opt in are completely unaffected.
• Collision safety: on a name collision the test's own file wins. A shared file that would
  overwrite a file the test already provides is skipped (with a warning), so a shared step can never
  silently clobber a test-specific one.