Stabilize render diagnostics timing tests on Windows

[Copilot]

The go1.25/windows-latest Actions job was failing on diagnostics tests that assumed extremely short renders would always produce non-zero durations. On Windows, those paths can legitimately measure as 0s, so the suite was asserting on timer resolution rather than behavior.

• Root cause

  • Several interpreter and VM diagnostics tests asserted EngineDuration, helper duration, or partial duration were strictly greater than zero.
  • Those assertions were flaky on Windows because the rendered work completed within clock granularity.

• Test stabilization

  • Updated the affected diagnostics tests in the root package and VM packages to perform a small, deterministic amount of work during render.
  • Added short delays inside test-only helpers and partial feeders so duration assertions continue to validate diagnostics plumbing without depending on sub-millisecond timing.

• Scope

  • Test-only change.
  • No production render or diagnostics logic changed.

ctx.Set("slow", func() string {
	time.Sleep(20 * time.Millisecond)
	return ""
})

out, err := rootplush.Render(`<%= slow() %><%= name %>`, ctx)
require.NoError(t, err)
require.NotZero(t, diagnostics.EngineDuration)

[paganotoni]

@Mido-sys I took copilot for a run on this one and it seems to have fixed the tests. Does this make sense? I did not update your PR because I cannot. Im expecting to close this PR once you confirm it makes sense.